BaseConn: let _read/_readline raise NotImplementedError

BaseConn: let _read/_readline raise NotImplementedError

This way, it's easier to understand the code, since these
functions aren't referenced without existing in BaseConn.

Also change has_input() to raise NotImplementedError instead
of trying to instantiate NotImplemented - the latter is just
a singleton to return from the rich comparison methods.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>