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 #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