Updated pants script to clean after a failed attempt to bootstrap

Testing Done:
I've run 'build-support/clean.sh' and had it encounter an Untranslatable... kind of error.

On the next invocation, I see the output "Last bootstrap failed, cleaning up .pants.d" and the next bootstrapping succeeded.

Bugs closed: 213

Reviewed at https://rbcommons.com/s/twitter/r/501/