Fix handling of unparsable range headers

Riak CS v1.4.2 returns a 416 status code for parseable but invalid Range
headers, but a 500 status when the header can't be parsed. This makes
the behavior consistent (always return the 416 status) by properly
handling the result of 'fail' from mochiweb_http:parse_range_request
Merge pull request #186 from nparry/range_fix_for_cs

Fix handling of unparsable range headers