Log metric that produced exception

To help diagnose issues such as those reported in
graphite-project/graphite-web#282

Backport from 80d111f8a270c5272089c280366f44a36d905eb4 on master branch.
Merge pull request #446 from jamesjuran/carbonlink_exception_backport

Backport to 0.9.x: Log metric that produced exception