patch 8.2.2266: Vim9: it can be hard to see where white space is missing

Problem:    Vim9: it can be hard to see where white space is missing.
Solution:   Mention the text where the error was seen. (closes #7580)
diff --git a/src/userfunc.c b/src/userfunc.c
index 5fab412..ed7d1d6 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -501,8 +501,8 @@
 		|| !IS_WHITE_OR_NUL(s[1])))
     {
 	*white_error = TRUE;
-	semsg(_(e_white_space_required_before_and_after_str),
-						    equal_arrow ? "=>" : "->");
+	semsg(_(e_white_space_required_before_and_after_str_at_str),
+					       equal_arrow ? "=>" : "->", bef);
 	return NULL;
     }
     return skipwhite(s + 1);