stitch: Throw Errors instead of strings
stitch: Throw Errors instead of strings

When strings are thrown, they don't have any metadata, which
results in the user not seeing a stack trace. This makes it
very difficult to debug exactly where the error came from,
so this commit changes all of the thrown strings in
bindings.js to Errors.

kklin/kelda,quilt/quilt,NetSys/quilt,kklin/kelda,NetSys/quilt,LuiseV/quilt,kelda/kelda,ejj/quilt-dev,ejj/quilt-dev,kklin/quilt,quilt/quilt,kayousterhout/quilt,kelda/kelda,LuiseV/quilt,quilt/quilt,kayousterhout/quilt,quilt/quilt,NetSys/quilt,kklin/kelda,kelda/kelda,kklin/kelda,kayousterhout/quilt,LuiseV/quilt,kklin/quilt,kklin/quilt,kelda/kelda,LuiseV/quilt,NetSys/quilt,ejj/quilt-dev,ejj/quilt-dev,kayousterhout/quilt,LuiseV/quilt,kklin/quilt,NetSys/quilt