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 not error is raised)

The spec here may need a bit of adjustment depending on what platform it is being run, but should be a good starting point.

(This also incorporates the #187)