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)")