Bring restkit RequestFailed a bit closer to front

I am getting some 503s from time to time. That means I have to go
all the way to importing restkit to handle these errors gracefully.
This is strictly my opinion but this makes it a little bit easier
to find these exceptions.

Catching the problem and reducing the stack trace to a minimum would
also make sense since these exceptions are never raised based on
something that happened inside couchdbkit or restkit.
Merge pull request #156 from ferrix/exception_improvement

Bring restkit RequestFailed a bit closer to front