ValidationError is explained very briefly in the source code (linked by Nijas Raphy) like this

class ValidationError(except_orm):
    """ Violation of python constraints
    Example: When you try to create a new user with a login which already exist in the db."""
    def __init__(self, msg):
        super(ValidationError, self).__init__(msg)

UserError is not really explained at all, but you can search the source code for examples (directly on github or do a "git grep UserError" if you have the repo locally which I highly recommend) and it will give you an idea.