Include useful diagnostic information in errors

[fitzgen]

Right now, we just give error messages like BB(Block(0)), meaning that there is some error somewhere in block 0.

If one enables logging, then one gets messages like "block 0 does not end in a terminator" or something like that.

It would be nice if we put that information in the actual error itself, so folks didn't have to enable logging to figure out what went wrong.

[cfallin]

Agreed -- this would be an easy starter PR if someone wants to take it on!