Add PATCH_CONTINUE_ON_FAIL to allow patching to go through all the
patches even if one fail.

This helps when upgrading ports with a large number of patches, like
www/chromium where having to fix one patch, re-do the patching, fix the
fallout, 800 times, is really painful.

This fixes the first attempt at this which would make ports with
multiple patches patching one file somewhat explode.