MetaFile/TargetFile: ensure securesystemslib errors are all handled

[jku]

Filing this so we can merge #1437 (Add hash and length verification to MetaFile and TargetFile) and can continue with this afterwards.

We need to review the errors that maybe thrown by securesystemslib in in Basefile._verify_hashes():

• do we want to handle all of them?
• should we just raise LengthOrHashMismatchError or something else?

My first instinct is that this is like verify_signatures(): the error messages may be interesting but from client POV what matters is that we cannot guarantee that hashes match.

cc @sechkova

[jku]

For reference #1435 is the PR to fix same issue in signature verification