Use http error code 422 in CheckView when a merge is not possible. Previously the response used the code 500 in this case. This is not useful because it's not an internal server error when the check whether a PDF file can be merged fails because that's the point of the CheckView. The code used now means "Unprocessable Entity" and fits better. Part of mpuls/issue6009

[aliases]
release = egg_info -RDb "" sdist --formats=gztar,zip bdist_egg bdist_wheel
release_upload = egg_info -RDb "" sdist --formats=gztar,zip bdist_egg bdist_wheel upload