upload: add connection exceptions to upload retry protocol

[ashwoods]

Addresses our intermittent upload issues as described that #1017 was initially trying to address.

Although we are mainly running into "connection reset by peer" exceptions, I've added connection timeouts, and I'd consider adding a sleep between retries.

[ashwoods]

Sorry, meant to draft this from the start, PR was meant as a rough proposal and there a few things that are still unclear:

• ~~the original code returns None on error codes (type return mismatch).~~
• we have been getting Connection errors, but retrying timeouts "might" be a good idea. (yes/no)?
• should we add a sleep to the retries?

will add a test and a few improvements asap.

[sigmavirus24]

This doesn't merge cleanly, doesn't solve the problem at the right layer, and it re-invents a wheel for which there are many good solutions. I'm closing this.