refactor(utils): allow optional errmsg in handle_allocate_error

[johnpaulalex]

Description of Changes

Add optional errmsg param to handle_allocate_error in src/utils/cam_abortutils.F90

Mirrors the fix in ESCOMP/CAM-SIMA#311.

Linked Issues

Fixes ESCOMP/CAM#1322

Description of Generative AI Usage

Google Antigravity was used to write the code and tests, followed by human-guided verification.

Answer-Changing & Restart Compatibility

• Answer Impact: Bit-for-Bit (B4B). Adds a new optional parameter that nobody uses yet, and which defaults to a no-op.
• Restart Space: No restart file or state variable impact.

Verification Performed

• AI built some adhoc tests to verify behavior with and without errmsg. Felt a little heavyweight to submit but they do pass.