Validate OBSDIR in MERGE_RFT_ERTOBS forward job

[andreas-el]

Hi - I just helped a user to debug crash of Ert forward job MERGE_RFT_ERTOBS - it was a simple typo which user would have managed to fix if job exited with a proper error message.

The issue was a typo in OBSDIR, but resulted in a cryptic error message.
If OBSDIR is found in this job, we should validate the existence for it and report back if not found.