Refactor run_global_ctxt.

It currently is infallible and uses `abort_if_errors` and
`FatalError.raise()` to signal errors. It's easy to instead return a
`Result<_, ErrorGuaranteed>`, which is the more usual way of doing
things.

src/librustdoc/lib.rs

@@ -787,7 +787,7 @@ fn main_args(
             gcx.enter(|tcx| {
                 let (krate, render_opts, mut cache) = sess.time("run_global_ctxt", || {
                     core::run_global_ctxt(tcx, show_coverage, render_options, output_format)
-                });
+                })?;
                 info!("finished with rustc");
 
                 if let Some(options) = scrape_examples_options {