introduce recipe_exception in result.proto

recipe_engine/result.proto

Index: recipe_engine/result.proto
diff --git a/recipe_engine/result.proto b/recipe_engine/result.proto
index 5affb08af1c2305f053309596c1af24fe888b90c..0a319816060a6fb0faac3cf624d9e32387e633e8 100644
--- a/recipe_engine/result.proto
+++ b/recipe_engine/result.proto
@@ -28,9 +28,12 @@ message Failure {
     // Step timed out.
     Timeout timeout = 2;
 
-    // Step threw an exception.
+    // Recipe engine threw an exception.
     Exception exception = 3;
 
+    // A recipe threw an exception.
+    RecipeException recipe_exception = 6;
+
     // Step accessed invalid step data.
     StepData step_data = 4;
 
@@ -39,13 +42,20 @@ message Failure {
   }
 }
 
-// An unexpected exception occured during execution. Caused by the builtin
-// Exception class.
+// Recipe engine raised an unexpected exception during execution.
+// Caused by the builtin Exception class.
 message Exception {
   // Traceback of an exception which occured.
   repeated string traceback = 1;
 }
 
+// A recipe raised an unexpected exception during execution.
+// Caused by the builtin Exception class.
+message RecipeException {
+  // Traceback of an exception which occured.
+  repeated string traceback = 1;
+}
+
 // A step timed out during its execution. Caused by StepTimeout in
 // recipe_api.py
 message Timeout {