Improve unit conversion error logging (NGWPC-7604)

[PhilMiller]

When there are failures to convert units between data providers and BMI model consumers, we want to be able to track down the issue quickly and efficiently. We also don't want to be flooded with repetitive warnings about the same failed conversion.

Additions

• Exception type representing the provider's failure to perform a requested conversion
• Set of conversion failures seen previously, specific to the pair of formulations, variables, and units involved

Changes

• Catch conversion failures in Bmi_Module_Formulation::get_value[s]() et al., add information, and rethrow rather than logging here
• Catch the enriched exception in Bmi_Module_Formulation::set_model_inputs_prior_to_update(), check whether that variety of failure has already been reported, and log if it has not.

Removals

• Logging in get_value which is now redundant with logging in the caller
• Non-specific conversion failure logging flags

Todos

• Currently unused get_values() call path isn't modified

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[PhilMiller]

The unit test failure is real - the change as it is right now breaks the tests for Bmi_Multi_Formulation

[cmaynard-ngwpc]

Reviewed latest commits. Looks good from what I can tell.