Rpc.start() now calls get_system_info() after launching the server
to verify it started successfully. If the server exits early (e.g.
due to an invalid accounts directory), the core error message from
stderr is captured and included in the raised JsonRpcError.
The reader_loop now unblocks pending RPC requests when the server
closes stdout, so callers never hang on a dead server.
Export JsonRpcError from the deltachat_rpc_client package.
Add test_early_failure verifying that Rpc.start() raises with
the actual core error message for invalid accounts directories.
