Gracefully abort on type incompatibility

Since the only use of `TypeCx::bug` is in `Constructor::is_covered_by`,
it is tempting to return `false` instead of `Err()`, but that would
cause "non-exhaustive match" false positives.

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -900,7 +900,7 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
         Ok(())
     }
 
-    fn bug(&self, fmt: fmt::Arguments<'_>) -> ! {
+    fn bug(&self, fmt: fmt::Arguments<'_>) -> Self::Error {
         span_bug!(self.scrut_span, "{}", fmt)
     }