Make Diagnostic::is_error return false for Level::FailureNote.

It doesn't affect behaviour, but makes sense with (a) `FailureNote` having
`()` as its emission guarantee, and (b) in `Level` the `is_error` levels
now are all listed before the non-`is_error` levels.

compiler/rustc_errors/src/diagnostic.rs

@@ -238,8 +238,7 @@ impl Diagnostic {
             Level::Bug
             | Level::DelayedBug(DelayedBugKind::Normal)
             | Level::Fatal
-            | Level::Error
+            | Level::Error => true,
-            | Level::FailureNote => true,
 
             Level::ForceWarning(_)
             | Level::Warning
@@ -248,6 +247,7 @@ impl Diagnostic {
             | Level::OnceNote
             | Level::Help
             | Level::OnceHelp
+            | Level::FailureNote
             | Level::Allow
             | Level::Expect(_) => false,
         }

compiler/rustc_errors/src/lib.rs

@@ -1541,6 +1541,8 @@ pub enum Level {
     ///
     /// The [`LintExpectationId`] is used for expected lint diagnostics. In all other cases this
     /// should be `None`.
+    ///
+    /// Its `EmissionGuarantee` is `()`.
     ForceWarning(Option<LintExpectationId>),
 
     /// A warning about the code being compiled. Does not prevent compilation from finishing.
@@ -1570,7 +1572,8 @@ pub enum Level {
     /// Its `EmissionGuarantee` is `()`.
     OnceHelp,
 
-    /// Similar to `Note`, but used in cases where compilation has failed. Rare.
+    /// Similar to `Note`, but used in cases where compilation has failed. When printed for human
+    /// consumption, it doesn't have any kind of `note:` label. Rare.
     ///
     /// Its `EmissionGuarantee` is `()`.
     FailureNote,