LGTM, but needs a test

Added tests. Let me know though if my assumption about Meta.lower not throwing exceptions (in a way that can be mocked here) is accurate. I looked to see if there was a way and there didn't seem to be.

A slight wrinkle here is that the eval frame will still show up in stacktraces. REPL.jl gets around that by calling a C function to do the eval, which are hidden normally. But that may not be worth trying to achieve?

Is it really that important? The fallback REPL isn't really supposed to be used regularly.