patch 8.2.2726: confusing error message with white space before comma

Problem:    Confusing error message with white space before comma in the
            arguments of a function declaration.
Solution:   Give a specific error message. (closes #2235)

src/testdir/test_vim9_func.vim

@@ -1263,6 +1263,15 @@ def Test_arg_type_wrong()
   CheckScriptFailure(['def Func5(items)', 'echo "a"'], 'E1077:')
 enddef
 
+def Test_white_space_before_comma()
+  var lines =<< trim END
+    vim9script
+    def Func(a: number , b: number)
+    enddef
+  END
+  CheckScriptFailure(lines, 'E1068:')
+enddef
+
 def Test_white_space_after_comma()
   var lines =<< trim END
     vim9script

src/userfunc.c

@@ -307,6 +307,17 @@ get_function_args(
 		emsg(_("E989: Non-default argument follows default argument"));
 		goto err_ret;
 	    }
+
+	    if (VIM_ISWHITE(*p) && *skipwhite(p) == ',')
+	    {
+		// Be tolerant when skipping
+		if (!skip)
+		{
+		    semsg(_(e_no_white_space_allowed_before_str_str), ",", p);
+		    goto err_ret;
+		}
+		p = skipwhite(p);
+	    }
 	    if (*p == ',')
 	    {
 		++p;

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2726,
 /**/
     2725,
 /**/