rx don't exit

rx don't exit

exiting in library code is antisocial. if rx encounters an error
which cannot be returned and would otherwise indicate a service thread
has gone away, assert.

Reviewed-on: http://gerrit.openafs.org/668
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>