[messages] Fix error message.

Fixes the error message emitted by
`MessageWithAnalyzerCode.decodeType` if the type of the YAML node is
not a string.

Change-Id: I6a6a6964a5ad3f5dad1913ec37b77ceecc0bd576
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465968
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: stereotype441

pkg/analyzer_utilities/lib/analyzer_messages.dart

@@ -729,6 +729,6 @@ LocatableDiagnostic $withArgumentsName({$withArgumentsParams}) {
     YamlScalar(:String value) =>
       AnalyzerDiagnosticType.fromString(value) ??
           (throw 'Unknown analyzer diagnostic type'),
-    _ => throw 'Must be a bool',
+    _ => throw 'Must be a string',
   };
 }