Update fees API for validation and error checking

[Shadouts]

The validation functions validateFee, validateFeeRate, checkFeeError, checkFeeRateError, and getFeeErrorMessage are not ideal in their function or naming. In previous versions, only validateFee and validateFeeRate existed. These returned an error message, but fee error types were added in v1.2.0 in order to allow for applications to decide their own path based on an error type. To maintain backward compatibility while providing error information, checkFeeError, checkFeeRateError, and getFeeErrorMessage were added.

For the next major version, the backward compatibility for the validateFee and validateFeeRate functions should be eliminated. Maybe they should return a different type the provides error messages, error type, and other info.

[jevidon]

Migrated to new repo: unchained-capital/caravan#374