[5pt] Handle system exits with FIM_exit_codes

[mluck]

A few scripts in fim_pipeline.sh handle abnormal conditions with sys.exit(), sys.exit(0), or sys.exit(1). These should be handled with FIM_exit_codes in order to prevent hidden downstream pipeline errors caused by missing files, etc. See #1212 for an example.

[RobHanna-NOAA]

Well.. yes, and there are lots of places we do want those errors. Hitting sys.exit (), (0) and (1) are ok if we really want the huc processing to fail that HUC. Other systems pick up the errors for sys.exit(1) and put it in the unit_errors folder. We really only use our special codes so we know how to catch them in key places and log them different (if at all). See examples via filter_catchments_and_add_attributes.py with the search code of FIM_exit_codes.NO_FLOWLINES_EXIST, then in proces_branch.sh and fim_process_unit_wb.sh, it catches status exist codes such as a 61 and decides what to do with them. We can add more codes if we want. Note the best system, but it works.