rustdoc: Use diagnostics for error when including sources

This error probably almost never happens, but we should still use the
diagnostic infrastructure. My guess is that the error was added back
before rustdoc used the rustc diagnostic infrastructure (it was all
`println!` and `eprintln!` back then!) and since it likely rarely occurs
and this code doesn't change that much, no one thought to transition it
to using diagnostics.

Note that the old error was actually a warning (it didn't stop the rest
of doc building). It seems very unlikely that this would fail without
the rest of the doc build failing, so it makes more sense for it to be a
hard error.

The error looks like this:

    error: failed to render source code for `src/test/rustdoc/smart-punct.rs`: "bar": foo
      --> src/test/rustdoc/smart-punct.rs:3:1
       |
    3  | / #![crate_name = "foo"]
    4  | |
    5  | | //! This is the "start" of the 'document'! How'd you know that "it's" ...
    6  | | //!
    ...  |
    22 | | //! I say "don't smart-punct me -- please!"
    23 | | //! ```
       | |_______^

I wasn't sure how to trigger the error, so to create that message I
temporarily made rustdoc always emit it. That's also why it says "bar"
and "foo" instead of a real error message.

Note that the span of the diagnostic starts at line 3 because line 1 of
that file is a (non-doc) comment and line 2 is a blank line.

src/librustdoc/html/sources.rs

@@ -54,12 +54,10 @@ impl DocFolder for SourceCollector<'_, '_> {
             self.scx.include_sources = match self.emit_source(&filename) {
                 Ok(()) => true,
                 Err(e) => {
-                    println!(
-                        "warning: source code was requested to be rendered, \
-                         but processing `{}` had an error: {}",
-                        filename, e
+                    self.scx.tcx.sess.span_err(
+                        item.span.inner(),
+                        &format!("failed to render source code for `{}`: {}", filename, e),
                     );
-                    println!("         skipping rendering of source code");
                     false
                 }
             };