Fail on REXML exceptions

Maruku uses REXML internally to parse the xml/html snippets. But if
the parsing fails, no error are generated and the task does not fail.

Let the REXML exceptions appear when processing a file so as to validate
html snippets also.
Merge pull request #476 from progit/fail_on_REXML_exception

Fail on REXML exceptions
Merge branch 'progit/master' into ja