More informative / less overwhelming server configuration errors

[marcua]

If you misconfigure ayb.toml, the error is informative, but the information is hidden in a long string:

called `Result::unwrap()` on an `Err` value: Other { message: "Unable to deserialize toml string: Error { inner: Error { inner: TomlError { message: \"missing field `max_snapshots`\", raw:...

The server should catch TomlError and print message to make the issue clearer.