Validate that all modifications are resolvable

[RalfG]

Instead of logging unmapped modifications

https://github.com/compomics/ms2rescore/blob/e3f723d9e90c2431c7627e4ffa8498522f4c847a/ms2rescore/parse_psms.py#L73-L77

check if all modifications are resolvable after mapping and report the ones that are not.