[8pt] Small CatFIM bugs in 4.8.7.2

[EmilyDeardorff]

In reviewing the most recent release of CatFIM, we have found the following bugs to be resolved:

• get_thresholds() function currently returns the count of thresholds available, but it doesn't differentiate between stages and flows available. Look into this and update if needed.
• Currently, CatFIM errors out with a slightly confusing message if no polygons are created (see below). Update the code so it gives a more descriptive exit message in that instance.
• CatFIM compare layers are missing the leading zeroes on the HUC.

Completed tasks:

• Site QBYQ2 is in located British Columbia. Resolve the issue by adding the site to the restricted sites list OR implementing a filter to remove non-US sites.
  -> Completed in PR 1636
• The CatFIM README.md file has gotten slightly out-of-date in regards to run commands. Review and update the doc as needed.
  -> Completed in PR 1636

>>>>>>>>>>>>>>>>>>>>>>>>>>>> 
Wrapping up processing HUCs for Stage-Based CatFIM... 
CRITICAL Traceback (most recent call last):
  File "/foss_fim/tools/catfim/generate_categorical_fim.py", line 1998, in <module>
    process_generate_categorical_fim(**args)
  File "/foss_fim/tools/catfim/generate_categorical_fim.py", line 266, in process_generate_categorical_fim
    generate_stage_based_categorical_fim(
  File "/foss_fim/tools/catfim/generate_categorical_fim.py", line 1703, in generate_stage_based_categorical_fim
    raise Exception(f"no csv files found - missing attribute CSVs in {attributes_dir}")
Exception: no csv files found - missing attribute CSVs in /data/catfim/emily_test/idaho_underflooding1_stage_based/attributes

These fixes should be completed for FIM 6.1.