added new ChecksumMismatchError which extends BagValidationError and includes a list of each failed checksum validation as a dictionary. fixes #30
doc fix
fixed stringification of ChecksumMismatchError
introduced ChecksumMismatch to bundle information about a given fixity verification error, rather than relying on a dictionary
libraryofcongress repo is now canonical
New classes for representing validation errors

Specifically, BagValidationFileError (for representing info
about an individual file that failed to validate) and
BagValidationErrorSet (for representing a collection of
error objects generated by a validation run).

These classes amend/replace the ManifestError and
ChecksumMismatch classes from recent commits.
Move BagValidationFileError types into classes

Additionally, change the way the errors get delivered.
renamed errors list to details ; simplified error/exception class hierarchy
validate does not work with --processes flag yet, see #2
renamed to make it look less like an exception, since it is not an exception
merged 30-provide-list-of-validation-errors. fixes #30