Return proper HTTP errors

[albertpastrana]

As an API client
I want the service to return proper http errors
so that I know if I have to fix my request or it's that the service is down

Acceptance Criteria
This just applies to the code we have for the POC (more may need to be added as the project evolves, to be defined in each new feature/change):

• Repository not found: 404
• File not found: 404
• Reference not found: 404
• No repo/file/reference param: 400
• Other errors: 500

[kittsville]

As per #24 reference will become an optional param, so we'll only need to return 400 for repo/file missing. Although if ${repo} was missing it'd just be localhost:7791/repo/, which could return a list of the available repos?

[nathankleyn]

@kittsville I converted the above list into a task list for you and moved it into the ticket description to make it a little easier to track — this causes it to show like subtasks elsewhere in the UI. 👍

See https://help.github.com/en/articles/about-task-lists for more info.

[nathankleyn]

Re-opening for now as there's still two errors to do above (it got closed automatically by #31).

[kittsville]

IIRC those are already done, as they were implemented in #31