minor improvements to README / error output

I ran into some bumps making use of JAPR as a non-ruby dev, and thought I'd offer some suggestions to smooth them out.

Basically, I needed to see the exception messages (why did this asset fail to process?) and thought I'd save the next person the time of sorting out using LESS.

One thing that came out of the troubleshooting: I had previously been using a YAML header in the misbehaving file (which the LESS compiler didn't care for).  I'd like to go back to doing that, so I'll be opening an issue.