Add a callback on Loader to signal when a Load starts

[colinkho]

This includes variables that captures the error and error count if it is starting a retry

[colinkho]

@icbaker I have incorporated the comments

[icbaker]

I'm going to send this for internal review now. You may see some more commits being added as I make changes in response to review feedback. Please refrain from pushing any more substantive changes as it will complicate the internal review - thanks!

[colinkho]

Apologies @icbaker , I accidentally updated my main branch and had newer commits rebased on top. Hope this doesn't complicate the internal review

[icbaker]

No worries, i've force pushed my own changes back.

[icbaker]

btw I think people often send PRs from not the main branch of their forks, possibly for this reason, but as an aside: using the main branch in your fork also slightly upsets GitHub's command line tool because we have to run:

$ gh pr checkout 1580 --branch pr-1580

Because the regular command will try to update our main branch directly, which we don't want (and often fails anyway):

$ gh pr checkout 1580

So for future PRs, it might be useful to create a new well-named branch in your fork for each one.

[colinkho]

Thanks. I didn't realize that the branch of the fork continuously updates the PR. I had thought it took a snapshot or merely cherry-picked the commits.

Also good info on the gh command line. I've been doing things the old-fashioned git way -- I'll try this out. Thank you!