Allow missing assets

This PR removes the inconsistency between Linux and OSX versions of wkhtmltopdf.

When there are missing assets (images, JS etc):

- on Linux (x64): wkhtmltopdf exits with status code = 2 and does produce actual PDF (but an error is raised)
- on OSX: wkhtmltopdf exits with status code = 0 and does produce actual PDF (and no error is raised)

This also means that no failures will occur during local development on OSX, but it is more likely to fail in production.

The spec here may need a bit of adjustment depending on what platform it is being run on, but should be a good starting point.

(This also incorporates the #187)