The Server has no notion of a plugin “package” in the sense of a directory containing individal plugin files. Internally, the Server just recursively iterates over the folders in the extension path(s) and tries to load everything that looks like a SC plugin.
OversamplingOscillators could have just packed all their plugins in a single binary, which is exactly what the core plugins are doing. (There is hardly any reason for shipping SC plugins as individual binaries.) Then you would only get a single error message.
You’re right that we don’t have to repeat the full error message over and over again. But this would only save us 2 of 4 lines. We have to include the first and last line because these are specific to each plugin.
In conclusion, I think the problem is really the OversamplingOscillators. I think the author has been new to SC plugin development and probably just wasn’t aware that a single plugin binary can contain multiple UGen classes. I would suggest to file an issue on their repo.