Fixes #1800. Add file name to NC4 asserts

[mathomp4]

Description

This PR adds to asserts in NetCDF4_get_var.H so that not only will the variable be output, but also the file in question.

Related Issue

Closes #1800

Motivation and Context

Makes the error message a bit more explicit helping to find bad files.

How Has This Been Tested?

Hard to test assertion failures. Maybe @lizziel has a case that triggers it?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)

Checklist:

• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog

[lizziel]

Looks good! Regarding testing, I'm not sure how the user files that triggered issues in NetCDF4_get_var.H were corrupted. Perhaps simply removing a required variable would work, or maybe that would get caught earlier on.