eval: Fix error messages from writefile

1. When calling writefile(list, fname, []) do not show error message twice.
2. Do not allow file name to be overwritten for writefile([1], 2).
3. Do not show “Can’t open file with an empty name” error after error like 
   “using Float as a String” when type of the second argument is not correct.
4. Do not give multiple error messages and still continue for code like 
   `writefile(["test", [], [], [], "tset"])`.

Note that to fix 4. ideally I need tv_check_str_or_nr which is currently present 
in two PRs: #6114 and #5119. I would want to avoid copying this function into 
a yet another PR.

Ref vim/vim#1476.

test/functional/eval/writefile_spec.lua

@@ -1,15 +1,34 @@
 local helpers = require('test.functional.helpers')(after_each)
+local lfs = require('lfs')
 
 local clear = helpers.clear
 local eq = helpers.eq
 local funcs = helpers.funcs
+local meths = helpers.meths
 local exc_exec = helpers.exc_exec
 local read_file = helpers.read_file
+local write_file = helpers.write_file
+local redir_exec = helpers.redir_exec
 
 local fname = 'Xtest-functional-eval-writefile'
+local dname = fname .. '.d'
+local dfname_tail = '1'
+local dfname = dname .. '/' .. dfname_tail
+local ddname_tail = '2'
+local ddname = dname .. '/' .. ddname_tail
 
-before_each(clear)
-after_each(function() os.remove(fname) end)
+before_each(function()
+  lfs.mkdir(dname)
+  lfs.mkdir(ddname)
+  clear()
+end)
+
+after_each(function()
+  os.remove(fname)
+  os.remove(dfname)
+  lfs.rmdir(ddname)
+  lfs.rmdir(dname)
+end)
 
 describe('writefile()', function()
   it('writes empty list to a file', function()
@@ -68,26 +87,54 @@ describe('writefile()', function()
     eq('a\0', read_file(fname))
   end)
 
+  it('shows correct file name when supplied numbers', function()
+    meths.set_current_dir(dname)
+    eq('\nE482: Can\'t open file 2 for writing: illegal operation on a directory',
+       redir_exec(('call writefile([42], %s)'):format(ddname_tail)))
+  end)
+
   it('errors out with invalid arguments', function()
-    eq('Vim(call):E119: Not enough arguments for function: writefile',
-       exc_exec('call writefile()'))
-    eq('Vim(call):E119: Not enough arguments for function: writefile',
-       exc_exec('call writefile([])'))
-    eq('Vim(call):E118: Too many arguments for function: writefile',
-       exc_exec(('call writefile([], "%s", "b", 1)'):format(fname)))
+    write_file(fname, 'TEST')
+    eq('\nE119: Not enough arguments for function: writefile',
+       redir_exec('call writefile()'))
+    eq('\nE119: Not enough arguments for function: writefile',
+       redir_exec('call writefile([])'))
+    eq('\nE118: Too many arguments for function: writefile',
+       redir_exec(('call writefile([], "%s", "b", 1)'):format(fname)))
     for _, arg in ipairs({'0', '0.0', 'function("tr")', '{}', '"test"'}) do
-      eq('Vim(call):E686: Argument of writefile() must be a List',
-         exc_exec(('call writefile(%s, "%s", "b")'):format(arg, fname)))
+      eq('\nE686: Argument of writefile() must be a List',
+         redir_exec(('call writefile(%s, "%s", "b")'):format(arg, fname)))
     end
-    for _, args in ipairs({'%s, "b"', '"' .. fname .. '", %s'}) do
-      eq('Vim(call):E806: using Float as a String',
-         exc_exec(('call writefile([], %s)'):format(args:format('0.0'))))
-      eq('Vim(call):E730: using List as a String',
-         exc_exec(('call writefile([], %s)'):format(args:format('[]'))))
-      eq('Vim(call):E731: using Dictionary as a String',
-         exc_exec(('call writefile([], %s)'):format(args:format('{}'))))
-      eq('Vim(call):E729: using Funcref as a String',
-         exc_exec(('call writefile([], %s)'):format(args:format('function("tr")'))))
+    for _, args in ipairs({'[], %s, "b"', '[], "' .. fname .. '", %s'}) do
+      eq('\nE806: using Float as a String',
+         redir_exec(('call writefile(%s)'):format(args:format('0.0'))))
+      eq('\nE730: using List as a String',
+         redir_exec(('call writefile(%s)'):format(args:format('[]'))))
+      eq('\nE731: using Dictionary as a String',
+         redir_exec(('call writefile(%s)'):format(args:format('{}'))))
+      eq('\nE729: using Funcref as a String',
+         redir_exec(('call writefile(%s)'):format(args:format('function("tr")'))))
     end
+    eq('TEST', read_file(fname))
+  end)
+
+  it('stops writing to file after error in list', function()
+    local args = '["tset"] + repeat([%s], 3), "' .. fname .. '"'
+    eq('\nE806: using Float as a String',
+        redir_exec(('call writefile(%s)'):format(args:format('0.0'))))
+    eq('tset\n', read_file(fname))
+    write_file(fname, 'TEST')
+    eq('\nE730: using List as a String',
+        redir_exec(('call writefile(%s)'):format(args:format('[]'))))
+    eq('tset\n', read_file(fname))
+    write_file(fname, 'TEST')
+    eq('\nE731: using Dictionary as a String',
+        redir_exec(('call writefile(%s)'):format(args:format('{}'))))
+    eq('tset\n', read_file(fname))
+    write_file(fname, 'TEST')
+    eq('\nE729: using Funcref as a String',
+        redir_exec(('call writefile(%s)'):format(args:format('function("tr")'))))
+    eq('tset\n', read_file(fname))
+    write_file(fname, 'TEST')
   end)
 end)