No indication of failure if we can't match an "announcement"

[meejah]

When asked to instantiate a storage-client in NativeStorageServer._make_storage_system there is no indication (in logs or otherwise) that an announcement failed to match.

This could be, for example, a typo in a plugin name.

In the particular case I'm looking at, there isn't an introducer at all just static servers in servers.yaml so the behavior a user sees is that one or more servers are configured, but nothing shows up in the Welcome page (nor in the logs).

Ideally the Welcome page would show some indication of the problem but logging something would be a useful first step.

[meejah]

​https://github.com/tahoe-lafs/tahoe-lafs/blob/be2c6b09f37b0ae6fbcbdf414a484b80c237c3f2/src/allmydata/storage_client.py#L765

[meejah]

One way to test this is to have a servers.yaml with one ZKAPAuthorizer-enabled server but spell the plugin name wrong in the yaml file.