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

syntax: glob
**/*.pyc
**/*.pyo
*.swp
.tmp*
*.egg-info
dist/*
build/*
lib/*
*.orig
*.log
.settings/*
data/*
storage/*
.project
.pydevproject
*.db
odfcast/odfcast.ini