worker/catacomb: fix LP 1590645
worker/catacomb: fix LP 1590645

Fix a logical race where the catacomb may not report the final error
from a worker. The fix is to wait for _both_ goroutines to exit before
letting the catacomb return.

perrito666/juju,dimitern/juju,kat-co/juju,howbazaar/juju,dooferlad/juju,gabriel-samfira/juju,bz2/juju,axw/juju,anastasiamac/juju,AdamIsrael/juju,mjs/juju,waigani/juju,AdamIsrael/juju,mikemccracken/juju,axw/juju,frankban/juju,bz2/juju,mikemccracken/juju,alesstimec/juju,perrito666/juju,perrito666/juju,kat-co/juju,howbazaar/juju,howbazaar/juju,macgreagoir/juju,dooferlad/juju,mjs/juju,marcmolla/juju,dooferlad/juju,ericsnowcurrently/juju,waigani/juju,mjs/juju,reedobrien/juju,frankban/juju,AdamIsrael/juju,waigani/juju,anastasiamac/juju,dooferlad/juju,ericsnowcurrently/juju,dimitern/juju,macgreagoir/juju,mikemccracken/juju,kat-co/juju,bz2/juju,frankban/juju,gabriel-samfira/juju,macgreagoir/juju,kat-co/juju,alesstimec/juju,mjs/juju,perrito666/juju,bz2/juju,gabriel-samfira/juju,axw/juju,ericsnowcurrently/juju,anastasiamac/juju,bac/juju,howbazaar/juju,gabriel-samfira/juju,howbazaar/juju,marcmolla/juju,alesstimec/juju,gabriel-samfira/juju,bac/juju,mjs/juju,axw/juju,bz2/juju,ericsnowcurrently/juju,marcmolla/juju,waigani/juju,dimitern/juju,frankban/juju,marcmolla/juju,dimitern/juju,mjs/juju,reedobrien/juju,perrito666/juju,AdamIsrael/juju,dimitern/juju,bac/juju,bac/juju,macgreagoir/juju,frankban/juju,mjs/juju,alesstimec/juju,reedobrien/juju,anastasiamac/juju,axw/juju,bac/juju,mikemccracken/juju,marcmolla/juju,mikemccracken/juju,alesstimec/juju,reedobrien/juju,anastasiamac/juju,reedobrien/juju,kat-co/juju,macgreagoir/juju,dooferlad/juju,ericsnowcurrently/juju