Refactor: Defer error logging until after all send retries are exhausted

[xjaner]

Whenever the Consumer fails to upload the events in the queue to the Segment backend, an error is logged:

analytics-python/segment/analytics/request.py

Line 57 in cb3e7ff

log.error(e)

I think that this line should be replaced with a warning message and defer the error logging until all send retries are exhausted.
Otherwise, it might seem that some events were definitely lost when that's not the case.

[MichaelGHSeg]

Done! The error will now only show up after retries run out, and extra logging can be seen by enabling debug or backoff logging.