Render custom view when 500 error is encountered. Attempts to fix #1628.

This is an initial attempt at fixing the 500 error issue. In
development, I had to disable :show_exceptions in the Sinatra configure
block. (This has been commented out for production.) I'm not sure if
this implementation will interfere with Bugsnag's capturing.
Merge pull request #1656 from tbloncar/tb-500-page

Render custom view when 500 error is encountered. Attempts to fix #1628.