buck2: improve an error message

Summary:
If we cancel local then try to download from RE and fail, we'll favor showing
the local executor's error, but that will not be very helpful because it'll
be reported as "Cancelled".

The more useful error to show is the RE executor's, which would have taken
the claim (causing the cancellation) then failed. So, let's just do that.

Now, obviously, the error output will leave open the question of "but *why did
this not retry locally*?", but really this behavior is a) subtle and b) rare,
there's just not going to be a good way to succinctly explain what went
wrong.

Reviewed By: themarwhal

Differential Revision: D47429620

fbshipit-source-id: 0f325e90a9b4d606702da897bcd294f60af5d254

Author: krallin

app/buck2_execute_impl/src/executors/hybrid.rs

@@ -295,7 +295,7 @@ where
             // For the purposes of giving users a good UX, if both things failed, give them the
             // local executor's error, which is likely to not have failed because of e.g.
             // sandboxing.
-            let (mut primary_res, secondary_res) = if is_retryable_status(&second_res) {
+            let (mut primary_res, mut secondary_res) = if is_retryable_status(&second_res) {
                 if first_priority > second_priority {
                     (first_res, second_res)
                 } else {
@@ -305,6 +305,14 @@ where
                 (second_res, first_res)
             };
 
+            // But if the first result was a cancelled result then we definitely don't want that.
+            if matches!(
+                &primary_res.report.status,
+                CommandExecutionStatus::Cancelled
+            ) {
+                std::mem::swap(&mut primary_res, &mut secondary_res);
+            }
+
             primary_res.rejected_execution = Some(secondary_res.report);
             primary_res
         } else {