Return a list of exceptions encountered during verification

[johnscancella]

Currently we throw an exception at the first problem we discover when verifying a bag: https://github.com/LibraryOfCongress/bagit-java/blob/0e4f80b969279c1a6e4f5037f3b9236bb6a253e3/src/main/java/gov/loc/repository/bagit/verify/BagVerifier.java#L151-L173

Instead we should return a list of errors in the case that there is more than one problem with a bag and let the user of this library decide how to deal with them.