Rollup merge of #115272 - RalfJung:miri-error-print, r=saethlin

miri/diagnostics: don't forget to print_backtrace when ICEing on unexpected errors

This should fix the missing output encountered [here](https://github.com/rust-lang/rust/issues/115145#issuecomment-1694334410).

r? `@saethlin`

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -273,7 +273,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 // dedicated error variants should be introduced instead.
                 assert!(
                     !error.kind().formatted_string(),
-                    "const-prop encountered formatting error: {error:?}",
+                    "const-prop encountered formatting error: {}",
+                    self.ecx.format_error(error),
                 );
                 None
             }