Refactor setup_intro_circ_auth_key() to make it simpler.

It now uses the 'goto err' pattern, instead of the fatal_unreached()
pattern. The latter pattern is usually used when there is a loop, but there is
no loop in this function so it can be simplified easily.
1 job for bug34084_044 in 11 seconds (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Test
failed build #556

00:00:11

 
Name Stage Failure
failed
build Test
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/asn/tor/.git/
Checking out c1598be1 as bug34084_044...

Skipping Git submodules setup
Executing "step_script" stage of the job script
$ apt-get update -qq
/bin/sh: eval: line 87: apt-get: not found
ERROR: Job failed: exit code 127