Similar to boost::scope_exit, but fewer include dependencies.

Give MPD clients access to the error condition.

Fixes memory leak after InputStream::Check() failure.

Remove that surprising rule.

Instead of stopping playback completely, only CANCEL the queued song
if necessary, and use the SEEK command to play the selected song.
SEEK will take care for current playback state.

This breaks on some standard library implementations.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Apparently, the std::system_error constructor appends strerror()
already.