Caused by a revert accident in commit
f2cdbeac

Closes https://github.com/MusicPlayerDaemon/MPD/issues/631

This reverts commit 58d7804d.  It
caused a use-after-free bug when Client::OnSocketError() was called
due to a failed write, e.g. if the output buffer was full.

release v0.21.14

The new Response instance in the `catch` block didn't have the
`command` attribute set, so the error response didn't indicate which
command had failed, which however is required in the MPD protocol.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/628

Clarify that those can't throw, preparing for the next commit.