Distinguish between error vs. warning for external macros

[jordy25519]

I often see compiler notes for warnings accompanied with this message:

   = note: this error originates in a macro outside this <...>

would it be more sensible and correct to say this _warning_ originates...?

Very superficial but I find seeing "error" in compilation logs catches my eye far too often and
its only a warning. Happy to submit PR if it is a valid change.

[estebank]

This can be easily added at

rust/src/librustc_errors/emitter.rs

Lines 850 to 871 in daa53a5

	fn fix_multispans_in_std_macros(&mut self,
	span: &mut MultiSpan,
	children: &mut Vec<SubDiagnostic>,
	backtrace: bool) {
	let mut spans_updated = self.fix_multispan_in_std_macros(span, backtrace);
	for child in children.iter_mut() {
	spans_updated |= self.fix_multispan_in_std_macros(&mut child.span, backtrace);
	}
	if spans_updated {
	children.push(SubDiagnostic {
	level: Level::Note,
	message: vec![
	("this error originates in a macro outside of the current crate \
	(in Nightly builds, run with -Z external-macro-backtrace \
	for more info)".to_string(),
	Style::NoStyle),
	],
	span: MultiSpan::new(),
	render_span: None,
	});
	}
	}

by looking at the Diagnostic::code or, if not available, theDiagnostic::level and changing the text accordingly.