in start_bundle_coupled fail on missing files/dirs

[orichters]

Purpose of this PR

• If you somehow misspecified the files / folders, start_bundle_coupled broke quite late with unclear message. Rather fail early.
• in output.R, if no folder contains gdx files (and mif if needed), fail and tell the user instead of printing this nonsense:

Please choose runs to be used for output generation:

1,a: all
2,p: Search pattern by regular expression...
3,f: Search by fixed pattern...

Type of change

• Bug fix

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• All automated model tests pass (FAIL 0 in the output of make test-coupled-slurm)