vcs: Do not pass the backend key into the scm app instances. We used the key as a value for a response header. Since this header is removed there is no need to pass the backend key araound.

vcs: Remove custom response header which contains backend key. We used this custom header to skip the error handling. This is no linger needed since the VCS middleware is moved above of the error handling.

error-handling: Remove if condition to check if the response is a vcs response. We moved the VCS middleware out of the error handling. Therefore we don't need to skip the error handling anymore.

wsgi-stack: Move the vcs middleware out of the error handling. Currently we are bypassing the error handling in case of responses from the vcs middleware with a custom response header. By moving it out of the error handling we don't need the custom header anymore.

tests: Add a way to deactivate exception re-raising in tests.

wsgi-stack: Add a more meaningful comment why we insert the error middleware.

wsgi-stack: Use the pylons error handling middleware.

wsgi-stack: Refactor the error handling view into a middleware class.

configs: updated default configs based on latest changes in control config templates

default-reviewers: add reason when manually adding reviewers after creating a PR. Fixes #4250