Show IRC server error messages when unexpected

Show IRC server error messages when unexpected

Handle ERROR replies from IRC servers, displaying the error message
in the status buffer.

Add method 'disconnectExpected' to CoreNetwork to expose when a
disconnect is expected.  As we're expecting a server error message
when issuing QUIT, hide it to avoid showing redundant information.

This fixes users getting disconnected by the server without any way
to find out why.

Resolves GH-238.