patch 9.0.0150: error for using #{ in an expression is a bit confusing

Problem:    Error for using #{ in an expression is a bit confusing.
Solution:   Mention that this error is only given for an expression.
            Avoid giving the error more than once. (closes #10855)
diff --git a/src/vim9script.c b/src/vim9script.c
index 14f36b4..e3cb992 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -183,9 +183,9 @@
     int
 vim9_bad_comment(char_u *p)
 {
-    if (p[0] == '#' && p[1] == '{' && p[2] != '{')
+    if (!did_emsg && p[0] == '#' && p[1] == '{' && p[2] != '{')
     {
-	emsg(_(e_cannot_use_hash_curly_to_start_comment));
+	emsg(_(e_cannot_use_hash_curly_to_start_comment_in_an_expression));
 	return TRUE;
     }
     return FALSE;