Improve check for paths as Hashes in CaptureOptions

[stevenday]

Hi,

Thanks for the awesome project!

This is my first PR and my Ruby is a little bit rusty, so please excuse any idiosyncrasies! I'm also running windows locally so I couldn't get the whole dev environment up and running to test everything before pushing, but I'll fix up whatever Travis finds.

Without knowing a whole lot about how this was supposed to be working, hopefully I've captured the intent of the code a bit better and fixed the issue reported in #536 at the same time.

I went for options.fetch so that it raised a KeyError, because I figured it'd make it more obvious that you're supposed to provide a path key if your path is a hash, rather than silently returning nil.

Fixes #536 and #466

[stevenday]

I just noticed that a related bug was present in the GalleryGenerator too, so I've added a fix for that in 0c39b03.

The code duplication suggests that the GalleryGenerator should use the CaptureOptions to parse the options, but I didn't want to make such a big change here just to fix this small bug.