Upgrade commoner build tool to v0.8.12.

Fixes #394.
Fixes #615.

When a module fails to build, `STDERR` now contains a line indicating which module it was. Also, the build terminates immediately when such errors occur, causing `jsx` to have a non-zero exit code, so that `grunt` will also terminate immediately.

cc @petehunt @zpao @arsfeld @fabiomcosta