Make the forkM failure fail more tidily.  Interface-file inconsistencies
give rise to failures in the IfM monad.  An error message is printed, but
up to now we've also said "The impossible happened, must be a GHC bug".
That's not true, though, it could just be messed up interface files.

So this commit still makes the compiler halt, but in a tidier, less
self-accusatory way.

Still to come: when original names in interface files mention the
package Id too, the error will become clearer still.

	MERGE to STABLE