Tweak error counting.

We have several methods indicating the presence of errors, lint errors,
and delayed bugs. I find it frustrating that it's very unclear which one
you should use in any particular spot. This commit attempts to instill a
basic principle of "use the least general one possible", because that
reflects reality in practice -- `has_errors` is the least general one
and has by far the most uses (esp. via `abort_if_errors`).

Specifics:
- Add some comments giving some usage guidelines.
- Prefer `has_errors` to comparing `err_count` to zero.
- Remove `has_errors_or_span_delayed_bugs` because it's a weird one: in
  the cases where we need to count delayed bugs, we should really be
  counting lint errors as well.
- Rename `is_compilation_going_to_fail` as
  `has_errors_or_lint_errors_or_span_delayed_bugs`, for consistency with
  `has_errors` and `has_errors_or_lint_errors`.
- Change a few other `has_errors_or_lint_errors` calls to `has_errors`,
  as per the "least general" principle.

This didn't turn out to be as neat as I hoped when I started, but I
think it's still an improvement.

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -818,7 +818,7 @@ impl<D: Deps> DepGraphData<D> {
             None => {}
         }
 
-        if let None = qcx.dep_context().sess().dcx().has_errors_or_span_delayed_bugs() {
+        if let None = qcx.dep_context().sess().dcx().has_errors_or_lint_errors_or_delayed_bugs() {
             panic!("try_mark_previous_green() - Forcing the DepNode should have set its color")
         }