Fix Sphinx crashes on parallel build with an extension which raises unserializable exception

sphinx/errors.py

@@ -71,10 +71,9 @@ class SphinxParallelError(SphinxError):
 
     category = 'Sphinx parallel build error'
 
-    def __init__(self, orig_exc, traceback):
-        self.orig_exc = orig_exc
+    def __init__(self, message, traceback):
+        self.message = message
         self.traceback = traceback
 
     def __str__(self):
-        return traceback.format_exception_only(
-            self.orig_exc.__class__, self.orig_exc)[0].strip()
+        return self.message