Debian: fail gracefully if modules can't be loaded on install

Debian: fail gracefully if modules can't be loaded on install

By registering an error-handler for the init script used
in openvswitch-switch.postinst and detecting if module insertion fails,
it is possible to avoid failure to install in the case where the
openvswitch_mod module is not available.

This is done without altering the behaviour that the start target
of the openvswitch-switch init script will fail if module insertion fails.

This patch also adds a friendly hint as as to why starting
openvswitch-switch has failed if it is due to failure to insert
the openvswtich_mod. This message is displayed as necessary both
on package install and other calls to the start target of the
init script.

[Ben Pfaff fixed up == to = in postinst]