Include useful diagnostic information in errors

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.

Start by locating the error formatting that produces `BB(Block(0))` and the logging code that emits messages such as "block 0 does not end in a terminator." Trace how these diagnostics are created and check existing error tests, if any. Done means the actual error includes the useful diagnostic information without requiring logging.

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