rpc: Ignore and log errors during cancel

Cancelling the RPC acceptors can sometimes result in an error about
a bad file descriptor.

As this is the shutdown sequence we need to continue nevertheless,
ignore these errors, log a warning and proceed.

Fixes #4352.
rpc: Add acceptors only when listening succeeded
Merge pull request #4358

6afa493 rpc: Add acceptors only when listening succeeded (Wladimir J. van der Laan)
33e5b42 rpc: Ignore and log errors during cancel (Wladimir J. van der Laan)