Make most lexer errors non-fatal

Most errors that arise in the lexer can be recovered from. This allows
for more than one syntax error to be reported at a time.

Author: lilyball

src/libsyntax/parse/comments.rs

@@ -291,7 +291,7 @@ fn read_block_comment(rdr: &mut StringReader,
         while level > 0 {
             debug!("=== block comment level {}", level);
             if is_eof(rdr) {
-                rdr.fatal("unterminated block comment".to_strbuf());
+                rdr.fatal("unterminated block comment");
             }
             if rdr.curr_is('\n') {
                 trim_whitespace_prefix_and_push_line(&mut lines,