Not properly configured backend sets unhelpful errors, file is still uploaded

In the D7 version, if the backend is not properly configured, PDF templates can still be uploaded, leading to an improperly parsed PDF template without any fields. The following error messages are set, but not particularly helpful:

FillPDF service:

Local service:

Local bridge:

Only pdftk seems properly validated on the settings page, so can't be set if not existing (though it may later fail for other reasons).

IMO, a non-working backend should stop PDF templates from being uploaded or at least should make sure no FillPDF Form is created and the uploaded file is not saved. Also, the error messages should be more helpful in pointing to the problem.