TLMigrateInstance: add error messagess during Exec

Currently the migration of an instance doesn't show any error until the
end. We add two messages that show better the progress:

node1# gnt-instance migrate -f instance5
Wed Nov  4 04:04:34 2009 Migrating instance instance5
Wed Nov  4 04:04:34 2009 * checking disk consistency between source and target
Wed Nov  4 04:04:35 2009 * switching node node3 to secondary mode
Wed Nov  4 04:04:35 2009 * changing into standalone mode
Wed Nov  4 04:04:35 2009 * changing disks into dual-master mode
Wed Nov  4 04:04:40 2009 * wait until resync is done
Wed Nov  4 04:04:41 2009 * preparing node3 to accept the instance
Wed Nov  4 04:04:41 2009 * migrating instance to node3
Wed Nov  4 04:04:51 2009 Migration failed, aborting
Wed Nov  4 04:04:51 2009 * switching node node3 to secondary mode
Wed Nov  4 04:04:51 2009 * changing into standalone mode
Wed Nov  4 04:04:51 2009 * changing disks into single-master mode
Wed Nov  4 04:04:57 2009 * wait until resync is done
Failure: command execution error:
Could not migrate instance instance5: Failed to migrate instance: Failed
to migrate instance instance5: …

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>