Report all stacktraces in verbose mode (#3938)

Previously these were swallowed (unlike the ones in black/__init__.py)

CHANGES.md

@@ -44,6 +44,8 @@
 
 - Black no longer attempts to provide special errors for attempting to format Python 2
   code (#3933)
+- Black will more consistently print stacktraces on internal errors in verbose mode
+  (#3938)
 
 ### _Blackd_
 

src/black/concurrency.py

@@ -9,6 +9,7 @@
 import os
 import signal
 import sys
+import traceback
 from concurrent.futures import Executor, ProcessPoolExecutor, ThreadPoolExecutor
 from multiprocessing import Manager
 from pathlib import Path
@@ -170,8 +171,10 @@ async def schedule_formatting(
             src = tasks.pop(task)
             if task.cancelled():
                 cancelled.append(task)
-            elif task.exception():
-                report.failed(src, str(task.exception()))
+            elif exc := task.exception():
+                if report.verbose:
+                    traceback.print_exception(type(exc), exc, exc.__traceback__)
+                report.failed(src, str(exc))
             else:
                 changed = Changed.YES if task.result() else Changed.NO
                 # If the file was written back or was successfully checked as