Ports without URLs should not raise an exception in update()

[eudoxos]

The following lines retrieve url list from a port, but fail when there is no URL. I think this is legitimate (I double-checked with GCGetNumPortURLs and should be handled gracefully. Think of a NIC with no camera attached, or USB transport layer momentarily without camera — not a reason to crash, just the state of things at that moment:

harvesters/src/harvesters/core.py

Lines 374 to 382 in b97fec3

	try:
	if len(port.url_info_list) > 0:
	url = port.url_info_list[0].url
	else:
	raise LogicalErrorException(
	'The target port does not hold any URL.')
	except GenTL_GenericException:
	return created_new_file, None

I suggest to say

except (GetnTL_GenericException, LogicalException):
   return create_new_file, None

which is handled gracefully by the caller.