Add more to the message.

testing/skia_gold_client/lib/src/errors.dart

@@ -55,7 +55,7 @@ final class SkiaGoldProcessError extends Error {
 /// Error thrown when the Skia Gold process exits due to a negative image.
 final class SkiaGoldNegativeImageError extends SkiaGoldProcessError {
   /// Creates a new [SkiaGoldNegativeImageError] from the provided origin.
-  /// 
+  ///
   /// See [SkiaGoldProcessError.new] for more information.
   SkiaGoldNegativeImageError({
     required this.testName,
@@ -68,5 +68,9 @@ final class SkiaGoldNegativeImageError extends SkiaGoldProcessError {
   final String testName;
 
   @override
-  String get message => 'Negative image detected for test: "$testName"';
+  String get message => 'Negative image detected for test: "$testName".\n\n'
+      'The flutter/engine workflow should never produce negative images; it is '
+      'possible that someone accidentally (or without knowing our policy) '
+      'marked a test as negative.\n\n'
+      'See https://github.com/flutter/flutter/issues/145043 for details.';
 }