eval: Fix max_min functions

Found two bugs:

1. Multiple unneeded error messages, vim/vim#1039.
2. Unformatted error string, vim/vim#1040.

test/functional/eval/minmax_functions_spec.lua

@@ -0,0 +1,51 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local eval = helpers.eval
+local clear = helpers.clear
+local funcs = helpers.funcs
+local redir_exec = helpers.redir_exec
+
+before_each(clear)
+for _, func in ipairs({'min', 'max'}) do
+  describe(func .. '()', function()
+    it('gives a single error message when multiple values failed conversions',
+    function()
+      eq('\nE745: Using a List as a Number\n0',
+         redir_exec('echo ' .. func .. '([-5, [], [], [], 5])'))
+      eq('\nE745: Using a List as a Number\n0',
+         redir_exec('echo ' .. func .. '({1:-5, 2:[], 3:[], 4:[], 5:5})'))
+      for errmsg, errinput in pairs({
+        ['E745: Using a List as a Number'] = '[]',
+        ['E805: Using a Float as a Number'] = '0.0',
+        ['E703: Using a Funcref as a Number'] = 'function("tr")',
+        ['E728: Using a Dictionary as a Number'] = '{}',
+      }) do
+        eq('\n' .. errmsg .. '\n0',
+           redir_exec('echo ' .. func .. '([' .. errinput .. '])'))
+        eq('\n' .. errmsg .. '\n0',
+           redir_exec('echo ' .. func .. '({1:' .. errinput .. '})'))
+      end
+    end)
+    it('works with arrays/dictionaries with zero items', function()
+      eq(0, funcs[func]({}))
+      eq(0, eval(func .. '({})'))
+    end)
+    it('works with arrays/dictionaries with one item', function()
+      eq(5, funcs[func]({5}))
+      eq(5, funcs[func]({test=5}))
+    end)
+    it('works with NULL arrays/dictionaries', function()
+      eq(0, eval(func .. '(v:_null_list)'))
+      eq(0, eval(func .. '(v:_null_dict)'))
+    end)
+    it('errors out for invalid types', function()
+      for _, errinput in ipairs({'1', 'v:true', 'v:false', 'v:null',
+                                 'function("tr")', '""'}) do
+        eq(('\nE712: Argument of %s() must be a List or Dictionary\n0'):format(
+            func),
+           redir_exec('echo ' .. func .. '(' .. errinput .. ')'))
+      end
+    end)
+  end)
+end