Avoid early returning Err() in parsers, so we could let the the caller
of the parsers to handle the rest of input, and return error if it requires
parsing entirely.

The point is let returning Err() stay inside input.try(), so we can count
on input.try() to restore the position when parsing invalid idents.

From gecko bug: Bug 1356134 (https://bugzilla.mozilla.org/show_bug.cgi?id=1356134)