Strip useless linebreak in the payload of send:exception events

`traceback.format_exception` adds a `\n` to each element of the resulting
list, so the message will always have a pointless `\n` at the end. Removing
it makes CSV reports that include the message easier to parse visually,
because there's no longer a linebreak in the middle of each line.

changelog.d/20240129_190242_roman_HEAD.md

@@ -0,0 +1,5 @@
+### Changed
+
+- The "message" field of the payload of send:exception events
+  no longer includes a trailing linebreak
+  (<https://github.com/opencv/cvat/pull/7407>)

cvat/apps/events/handlers.py

@@ -518,7 +518,7 @@ def handle_rq_exception(rq_job, exc_type, exc_value, tb):
     tb_strings = traceback.format_exception(exc_type, exc_value, tb)
 
     payload = {
-        "message": tb_strings[-1],
+        "message": tb_strings[-1].rstrip("\n"),
         "stack": ''.join(tb_strings),
     }
 
@@ -563,7 +563,7 @@ def handle_viewset_exception(exc, context):
             "content_type": request.content_type,
             "method": request.method,
         },
-        "message": tb_strings[-1],
+        "message": tb_strings[-1].rstrip("\n"),
         "stack": ''.join(tb_strings),
         "status_code": status_code,
     }