Bogus error messages "BUG in SwiftNIO, unleakable promise leaked"

[weissi]

If a user accidentally shuts down the ELG before actually being done, they often get

BUG in SwiftNIO (please report), unleakable promise leaked.:486: Fatal error: leaking promise created at (file: "BUG in SwiftNIO (please report), unleakable promise leaked.", line: 486)

which is both

1. wrong (in all cases that I've seen)
2. Misleading

the message should be adjusted to say that it's likely that the EventLoop (which we can actually print!) was likely shut down already (and that it's a bug in NIO if it isn't shut down)