Make ICE an error and use a sensible error message

Author: bossmc

src/libsyntax/ext/tt/macro_rules.rs

@@ -302,8 +302,8 @@ fn check_lhs_nt_follows(cx: &mut ExtCtxt, lhs: &NamedMatch, sp: Span) {
             tt @ &TtSequence(..) => {
                 check_matcher(cx, Some(tt).into_iter(), &Eof);
             },
-            _ => cx.span_bug(sp, "wrong-structured lhs for follow check (didn't find \
-            a TtDelimited or TtSequence)")
+            _ => cx.span_err(sp, "Invalid macro matcher; matchers must be contained \
+               in balanced delimiters or a repetition indicator")
         },
         _ => cx.span_bug(sp, "wrong-structured lhs for follow check (didn't find a \
            MatchedNonterminal)")