Print all errors returned by the Percy API

[timhaines]

This prevents api response errors from being hidden when looking for response[data] when it doesn't exist. It catches the error earlier, and raises it.

This is perhaps an interim solution to getting all errors logged correctly. Later we may want to add some retries for specific types of errors.

Thanks @criddyp for the original commit on this.

[fotinakis]

A good interim solution that exposes more info rather than just dying. Later on, we'd want to replicate the behavior of the other client libs to actually swallow the errors, log them out as warnings, and disable Percy internally without raising an exception so that CI suites aren't affected.