pr: Return proper merge response if target reference is missing.

pr: Move log statement to allow early return and still log merge response.

vcs: Add enumeration for pull request update failure reasons.

pr: Add error messages for new merge failure reasons.

vcs: Add merge failure reasons for missing target ref and missing source ref.

login: Add some tests to check captcha setting retrieval. #4279

login: Make register views more robust if some POST parameters are missing. We fail to delete passsword/password confirm parameters if they are not part of the POST parameters. But failing to delete them if they are not present seems wrong. Better silently ignore if they are not present.

login: Start error message with uppercase letter. All other error messages are starting with uppercase and lowercase just looks wrong in the UI

login: Removed superfluos if condition.

login: Fix recaptcha. #4279 There was an error in the retrieval of private/public key from settings model. This leads to `None` as private key and this indicates that recaptcha is not active. Therefore it was never used no matter if private key was set or not.