Don't crash if we fail to read the job ad

[matyasselmeci]

We read and parse the job ad file in main.go:parse_job_ad() for getting Owner and ProjectName info of the job. It's not currently used (usage was removed in #50) but we plan to use it for telemetry in #41.

That function crashes (log.Fatal) if $_CONDOR_JOB_AD is set to a non-existent file, or if the job ad file exists but can't be read for some reason. This results in the job going on hold with the error message stash_plugin exited with code 1, unable to open output file .../.stash_plugin.out since the plugin doesn't write the output classad.

Don't exit out, just give a warning.