scripts: correctly warn when the local tarballs directory does not exist

Warn about a missing local tarball directory, only if it was configured.
Avoid the spurious message:
Directory '${CT_LOCAL_TARBALLS_DIR}' does not exist.
Will not save downloaded tarballs to local storage.

Thanks to "Pedro I. Sanchez" <psanchez@colcan.ca> for pointing out the
issue:
http://sourceware.org/ml/crossgcc/2009-12/msg00011.html

reporter_name="YEM"

reporter_url="http://ymorin.is-a-geek.org/"

reporter_comment=""