Remove all threading through of ErrorGuaranteed from the driver

It was inconsistently done (sometimes even within a single function) and
most of the rest of the compiler uses fatal errors instead, which need
to be caught using catch_with_exit_code anyway. Using fatal errors
instead of ErrorGuaranteed everywhere in the driver simplifies things a
bit.

src/bin/miri.rs

@@ -289,7 +289,8 @@ fn run_compiler(
     let exit_code = rustc_driver::catch_with_exit_code(move || {
         rustc_driver::RunCompiler::new(&args, callbacks)
             .set_using_internal_features(using_internal_features)
-            .run()
+            .run();
+        Ok(())
     });
     std::process::exit(exit_code)
 }