fix(source): Source giving E484 & parsing error at line 1 for lua files

It's happening because do_source is only expected to return FAIL when it
was unable to open file . But `nlua_exec_file` returns fail for parsing
and execution error too . Those errors are emitted through `nlua_error`.

So now return value of nlua_exec_file is ignored like do_cmdline. It now
only returns fail when it was unable to open file that check is done
before calling nlua_exec_file or do_cmdline. Errors in nlua_exec_file
are still directly emitted through nlua_error like before.

test/functional/ex_cmds/source_spec.lua

@@ -9,6 +9,8 @@ local feed_command = helpers.feed_command
 local write_file = helpers.write_file
 local exec = helpers.exec
 local eval = helpers.eval
+local exec_capture = helpers.exec_capture
+local neq = helpers.neq
 
 describe(':source', function()
   before_each(function()
@@ -90,4 +92,27 @@ describe(':source', function()
     eq(12, eval('g:c'))
     os.remove(test_file)
   end)
+
+  it("doesn't throw E484 for lua parsing/runtime errors", function()
+    local test_file = 'test.lua'
+
+    -- Does throw E484 for unreadable files
+    local ok, result = pcall(exec_capture, ":source "..test_file ..'noexisting')
+    eq(false, ok)
+    neq(nil, result:find("E484"))
+
+    -- Doesn't throw for parsing error
+    write_file (test_file, "vim.g.c = ")
+    ok, result = pcall(exec_capture, ":source "..test_file)
+    eq(false, ok)
+    eq(nil, result:find("E484"))
+    os.remove(test_file)
+
+    -- Doesn't throw for runtime error
+    write_file (test_file, "error('Cause error anyway :D')")
+    ok, result = pcall(exec_capture, ":source "..test_file)
+    eq(false, ok)
+    eq(nil, result:find("E484"))
+    os.remove(test_file)
+  end)
 end)