Improve error handling and messages

[RussellManser-NCO]

What is wrong?

The following scripts need improved error handling

• scripts/exaigfs_post.sh - explicitly check for grib2 file and include a descriptive message if it is missing
• scripts/exaigfs_prep.sh - call err_exit with a descriptive error message if the target input file is missing
• ush/ python scripts - should print "FATAL ERROR: " with descriptive message for all errors and "WARNING: " for all non-fatal errors

What should have happened?

Error handling with descriptive messages is required for any missing input files (General Application Standards)

Steps to reproduce

N/A

Bugzilla issue

No response

Additional information

No response

Do you have a proposed solution?

No response