Only sniff for errors when we don't get a checkstyle file back.
Only sniff for errors when we don't get a checkstyle file back.

When the output doesn't contain an XML prolog we can more safely assume
that eslint failed and can start the error sniffing.

This solves problems where lint rules can contain 'Cannot find module'.

markstory/lint-review,markstory/lint-review,markstory/lint-review