We should probably restructure the validation process

We should probably restructure the validation process
- Once a validation has started, the service should create a `processing` badge for the commit and return, so the user does not have to wait until cloning and validation is done (which might take some time).
- Currently a validation process never returns an error. If something goes wrong during the validation an error should be returned here; if an error is present, the service should then replace the `processing` badge with an appropriate `error` badge. If no error is present it is assumed that the build went through and an appropriate badge has been created by the validation process.

_Originally posted by @mpsonntag in https://github.com/G-Node/gin-valid/pull/33_