Feature/clean backtraces

Rack-test-rest assertions now clean their own backtraces so instead of seeing failures that point to RTR, they point to the line that is calling into RTR's methods.

Also cleaned up several exception states to put out more useful debugging info by default and refactored to DRY some things up.

NOTE: there's a version bump in this because I accidentally published off my repo earlier, will bump/publish again after this merge.