feature-request: raise an error in case a field is not matched.

[iamKunal]

Raise an error in case a field is not matched.

Let's say that there are missing fields in target that were not mapped, there should be an option that raises an error on missing mapping.

Refer https://github.com/switchupcb/copygen/issues/41. Goverter does this out of the box. It also has an option for ignoring non-exported fields, so it helps prevent false positives especially when mapping protos