Add metadata property to Loader function calls.

Pass options on metadata.traceurOptions.
This aligns us with spec changes for metadata and with es6-module-loader.

Refactor InternalLoader.load/load_ to simplify calls.
Clean up some error-usability issues from MultipleErrors change.

TBR=@arv
I removed the try/catch and change the Error in ModuleVisitor to say it is an InternalError.  I was hitting the ModuleVisitor path while working on the PR, but no test touches it.