Return reason for errors in REST API

[pablochacin]

When a call to the build service fails, the client gets a response with an Error field containing the description of the error.
As there may be multiple sources for the error, it would be convenient to categorize errors as related to the requirements from those related to the build service itself, and complement with a detailed reason for this error.

For example,
Error: "could not satisfy dependency", Reason: "unknown dependency"
Error: "build failed", Reason: "build timeout".

The API client library can wrap the reason into the build service error to allow its users to unwrap it.

[pablochacin]

Closed by #61