Kernel Invocation Error Reporting

[gilbo]

Right now, any error that happens while a kernel is running will cause a stack dump to report the error as originating within our compiler. That's bad for us.

There are two stages to fixing this:

1. Wrap all kernel invocations in a Lua xpcall() that produces a slightly more useful stack dump. That is, the stack dump should locate the error at the kernel call site instead.
2. Eventually, we'd like to plumb line-number information from the parser through to the Terra code-gen and then somehow have terra dump more useful debug information. This may be difficult and requires Zach's input.