Print helpful information for certain ExtData problems.

[mmanyin]

Description

Added info in _ASSERT messages to aid in debugging ExtData.yaml files.

Related Issue

Motivation and Context

These edits help users resolve problems in ExtData yaml specifications, with
more informative messages. This is by no means an exhaustive review of the code,
it is just a set of ASSERT statements that was accumulated in the process of
debugging a large hand-crafted yaml specification, during which MAPL would
terminate the model without providing a clue as to which section of yaml was problematic.

How Has This Been Tested?

This was tested using a standard PCHEM configuration of the GCM.
It is zero-diff except when MAPL encounters certain fatal errors, in which case it
prints a little more information in the slurm log.

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)
• Additional diagnostic information

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