Merge Duplicates spins forever when the duplicate search crashes

Describe the bug

FindPotentialDuplicates returns Ok() and does the work on a discarded task.
GetDuplicatesThenSignal sends CombineHub.MethodSuccess on success and nothing on
failure, so a crashed search leaves the user on a spinner forever — MergeHub waits on
that signal and SignalRHub has no timeout. #4366 logs the crash; it does not end the wait.

https://github.com/sillsdev/TheCombine/blob/master/Backend/Controllers/MergeController.cs#L178-L189

LiftController sends CombineHub.MethodFailure from its equivalent catch and SignalRHub
already handles it, but two things stop this being a one-liner:

1. MergeHub's failureAction is only setDataLoadStatus(Failure), which leaves
   IdenticalDuplicatesDialog claiming duplicates were found, Cancel hidden, over a
   GoalStatus.Loading spinner. It needs the alert and router.navigate(Path.Goals) that
   the request-failure path already does:
   https://github.com/sillsdev/TheCombine/blob/master/src/goals/Redux/GoalActions.ts#L186-L190

2. MethodFailure carries only a user id and GetAndStorePotentialDuplicates returns a bare
   bool, so a request that throws after a newer one started would end the newer one's wait.
   The counter that knows is private to MergeService:
   https://github.com/sillsdev/TheCombine/blob/master/Backend/Services/MergeService.cs#L411-L424

To Reproduce

Make MergeService.GetPotentialDuplicates throw, then work through every
identical-vernacular set in Merge Duplicates. The "searching for similar duplicates" dialog
never changes.

Expected behavior

The user is told the search failed and returned to Goals, as a failed findSimilarDuplicates
request already does. A superseded request stays silent.

Environment:

• Backend, any deployment; not browser-specific.

Start with MergeController.cs lines 178-189 and MergeService.cs lines 411-424 to trace duplicate-search failures and request supersession. Then follow MergeHub's failureAction and the request-failure path in src/goals/Redux/GoalActions.ts lines 186-190. Done means failed searches alert the user and return to Goals, while superseded requests do not end a newer wait.