patch 9.0.0299: error messages for setcmdline() could be better

Problem:    Error messages for setcmdline() could be better.
Solution:   Use more specific error messages. (Yegappan Lakshmanan,
            closes #10995)
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 1644de8..58dc152 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -4272,11 +4272,9 @@
 {
     int pos = -1;
 
-    if (argvars[0].v_type != VAR_STRING || argvars[0].vval.v_string == NULL)
-    {
-	emsg(_(e_string_required));
+    if (check_for_string_arg(argvars, 0) == FAIL
+	    || check_for_opt_number_arg(argvars, 1) == FAIL)
 	return;
-    }
 
     if (argvars[1].v_type != VAR_UNKNOWN)
     {