buffer: fix not return on error

Throwing a JS error from C++ does not mean the function will return early. This must be done manually.

Also remove extraneous comment no longer relevant.

Fix: 2903030c "buffer: switch API to return MaybeLocal"

Note: There is no way to add a test for this.

CI: https://jenkins-iojs.nodesource.com/job/iojs+any-pr+multi/178/

/cc @rvagg