Correct error message in umdp3_fixer.py

[t00sa]

The error message from umdp3_fixer.py still references fcm - see example below. This is misleading and needs updating.

Traceback (most recent call last):
  File "~/cylc-run/misc_opts/run1/work/1/script_azspice_umdp3_fixer/rosestem_branch_checker.py", line 186, in <module>
    main()
  File "~/cylc-run/misc_opts/run1/work/1/script_azspice_umdp3_fixer/rosestem_branch_checker.py", line 179, in main
    diff_cwd_working(model_source, path, saved_umask, tmpdir)
  File "#/cylc-run/misc_opts/run1/work/1/script_azspice_umdp3_fixer/rosestem_branch_checker.py", line 95, in diff_cwd_working
    raise ValueError(
ValueError: Ran fcm diff command and changes were made by the umdp3_fixer.py script.
[FAIL] python3 rosestem_branch_checker.py --source "$PWD" --col 80 # return-code=1