UnixSocketConnection handle remote close

[bjoernricks]

When trying to send a command to a UnixSocketConnection closed e.g. by gvmd after an error response a IOError is returned. In this case the socket must be re-opened. Maybe it would also be possible to check if the connection is closed.

Example code for catching such an exception

except IOError as e:
    if e.errno == errno.EPIPE: