Merge pull request #91 from PrivateStorageio/83.better-misconfiguration-failure

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/tests/_exception.py

src/_zkapauthorizer/tests/foolscap.py

src/_zkapauthorizer/tests/matchers.py

src/_zkapauthorizer/tests/test_foolscap.py

src/_zkapauthorizer/tests/test_plugin.py

src/_zkapauthorizer/tests/test_storage_protocol.py