Provide a more concise error for complete failures

[Chaphasilor]

If there are e.g. permission issues for an OD, ODD will output

URLs with errors:
<root OD URL>

at the end.
This indicates that not only didn't it find any files, but it also failed to actually scan the OD in the first place

[Chaphasilor]

The above output could indicate one of the following:

• The URL isn't an OD (and contains no links at all)
  Examples: 404 page, 502 page
• The URL is protected via e.g. a password (HTTP basic, etc.) and returns an 4xx status code
• The server sent no response at all

[Chaphasilor]

Warning for non-OD:

WARN OpenDirectoryIndexer..ctor [Processor 1] Error Response status code does not indicate success: 404 (Not Found). retrieving on try 1 for url '/stuff/Videos/'. Skipping..

Warning for non-success code (after a few retries, 502 error in this case):

WARN OpenDirectoryIndexer.TimerStatistics_Elapsed
...
WARN OpenDirectoryIndexer..ctor [Processor 1] Cancelling on try 5 for url 'xxx'

Warning for server not responding:

WARN OpenDirectoryIndexer..ctor [Processor 1] Error A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. (<domain>) retrieving on try 2 for url 'xxx'. Waiting 4 seconds
...
WARN OpenDirectoryIndexer..ctor [Processor 1] Cancelling on try 5 for url 'xxx'