Catch exceptions from service handler and return internal error

[bgreni]

• Catch exceptions from service handler and return 500 error
• Fix integration test and add test case
• Contributes to Better error handling #3

[bgreni]

@saviorand FYI this is a pretty harmless change so I'm going to go ahead and merge it. Just prevents the server from crashing if something goes wrong in the handler and allows users to create a 500 response by just raising an Error which I think is the intuitive behaviour.

[saviorand]

Nice! Also prints in integration tests are helpful. The only feedback I have is I found a typo in IntegerationTestService (should be IntegrationTestService) , also I think it's good to call INTERNAL_ERROR alias just INTERNAL_SERVER_ERROR because these aliases correspond to HTTP codes anyway. I can fix those two separately though