api: improve status code reporting

[fsouza]

Currently, gandalf has poor status code reporting. Almost all errors are reported as 400 - bad request, we need to improve it so we have:

• 400 - Bad request for validation errors (e.g.: missing or wrong parameters)
• 404 - Not found for entities that doesn't exist (repository, user and key)
• 409 - Conflict for entities that already exist (repository, user and key)
• 500 - Internal server error for internal errors in the server (e.g: error communicating with the database or marshalling the output in JSON format)

What's interesting is that the fake server in the gandalftest package already behaves like that.