the package version that fixes a given crash.

        Return None if the crash is not yet fixed, or an empty string if the
        crash is fixed, but it cannot be determined by which version. Return
        'invalid' if the crash report got invalidated, such as closed a
        duplicate or rejected.

        This function should make sure that the returned result is correct. If
        there are any errors with connecting to the crash database, it should
        raise an exception (preferably IOError).r