Log format of service exception handler

[destroy314]

When an exception raised in callback function of a service ,this function will log it to rosout:

ros_comm/clients/rospy/src/rospy/impl/tcpros_service.py

Lines 581 to 582 in 842f0f0

	def error_handler(self, e, exc_type, exc_value, tb):
	logerr("Error processing request: %s\n%s" % (e, traceback.format_exception(exc_type, exc_value, tb)))

However the return value of traceback.format_exception() is a list which cause it's hard to read the traceback message:

[ERROR] [1680007708.746949]: Error processing request:['Traceback (most recent call last):\n', '  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 633,in _handle_request\n    response = convert_return_to_response(self.handler(request), self.response_class)\n', ...

Can we use ''.join() or traceback.format_exc() like the code below to get a better format?

ros_comm/clients/rospy/src/rospy/topics.py

Line 753 in 842f0f0

logerr("bad callback: %s\n%s"%(cb, traceback.format_exc()))