What to do if --mets is unspecified and no mets.xml is present

[kba]

Currently, a processor call, if --mets was not given and no mets.xml is present in $PWD, the --help message is given. The reasoning is that if a user calls a processor without any arguments, it makes more sense to provide general help instead of the very specific error message mets.xml could not be found.

However from feedback from @dla-kramski, @ElexaDigitalia_twitter and others, this behavior (in core since 2.12.0 #438 OCR-D/spec#156) is confusing because it is also triggered if -I and -O are provided.

So I propose to change the behavior to this:

• If no options/arguments at all are provided to a processor, then print the --help
• If METS cannot be found, raise an error saying so

[bertsky]

So I propose to change the behavior to this:

* If no options/arguments _at all_ are provided to a processor, then print the `--help`

* If METS cannot be found, raise an error saying so

Looks good. (But for things that might go wrong in scripts/makefiles, I'd also make the first case exit with non-zero besides showing the help text.)