Fixes the issue whith AccessTokenResponse

Previously AccessTokenResponse didn't detect that the content of the
response where correct but started to search for a matching interaction.

This was due to a missing return-statement in handle_result
(oauth2test/base.py:123).
Reverts previous error handling

As FatalError not always means that we have a runtimeerror it seems we
shouldn't just dump a stacktrace to stdout when we receive one.

Probably should change the errorhandling to use another exception and
dump a stacktrace on that error instead.
Test for returned error on second usage of code
Merge pull request #7 from asheidan/feature/reject2ndcode

Test for rejecting second usage of the same authorization code