Handle blank description in _print_goals_help

[gshuflin]

If there is no description for a scope_info goal, print out a message to this
effect associated with that goal rather than just a blank line.

[jsirois]

This will probably need re-base to re-trigger a full CI and get past the current dup binary issues.

[jsirois]

I'm babysitting the red - apparently flake / timeout - shards. Will merge on green.

[jsirois]

@gshuflin this will need another rebase to kick off a fresh CI, getting:

...
2.19s$ ./build-support/bin/get_ci_bootstrapped_pants_pex.sh ${BOOTSTRAPPED_PEX_BUCKET} ${BOOTSTRAPPED_PEX_KEY_PREFIX}.${BOOTSTRAPPED_PEX_KEY_SUFFIX}
  
Error: Found 0 versions for s3://ci-public.pantsbuild.org/25831/572381372/pants.pex.py36.osx
The command "./build-support/bin/get_ci_bootstrapped_pants_pex.sh ${BOOTSTRAPPED_PEX_BUCKET} ${BOOTSTRAPPED_PEX_KEY_PREFIX}.${BOOTSTRAPPED_PEX_KEY_SUFFIX}
  " failed and exited with 1 during .
...