During bootstrapping, the script's name is not necessarily set.
During bootstrapping, the script's name is not necessarily set.

So check whether the name is actually set before printing it when
throwing an exception.

BUG=none
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23283011

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@16233 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh