patch 8.2.0404: writefile() error does not give a hint

Problem:    Writefile() error does not give a hint.
Solution:   Add remark about first argument.

src/filepath.c

@@ -1929,7 +1929,8 @@ f_writefile(typval_T *argvars, typval_T *rettv)
     }
     else
     {
-	semsg(_(e_invarg2), "writefile()");
+	semsg(_(e_invarg2),
+		_("writefile() first argument must be a List or a Blob"));
 	return;
     }
 

src/testdir/test_writefile.vim

@@ -16,6 +16,8 @@ func Test_writefile()
   call assert_equal("morning", l[3])
   call assert_equal("vimmers", l[4])
   call delete(f)
+
+  call assert_fails('call writefile("text", "Xfile")', 'E475: Invalid argument: writefile() first argument must be a List or a Blob')
 endfunc
 
 func Test_writefile_ignore_regexp_error()

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    404,
 /**/
     403,
 /**/