Add some test to try report issue #470

Add some test with notice send with empty backtrace
Add some test directly in the ErrorReport model
Return an 422 HTTP status code in notice submit when API_KEY is not valid

When a notice is do with an invalid api_key the return is a 404 HTTP
code and no explain about what really happen. Now if you try post a
notice with a bad API_KEY, errbit return a 422 HTTP status code, like
doing by airbrake.

see #472
Extract test from spec/model/app to spec/model/error_report

Some test on spec/model/app are too specific to spec/model/error_report
move to this spec
Extract completly the ErrorReport class

Extract all concern about this call to his own class and move test to
this class
Extract some test from controller/noticess to error_report
Merge pull request #475 from shingara/features/issue_472

Return 422 HTTP status code when invalid API key