Better error handling in backfill script

Fix error handling for backfill

defer the channel close so that the channel is closed no matter what
always report parse and insert errors
handle a cancelled context in the sql query
clean up inappropriate log.Fatal calls leftover from an earlier refactor
don't print success if insertion failed

Abort backfill after limited errors

If there is a persistent problem with one of the network connections, continuing to attempt the insertion is a waste of time, and if the list of errors gets too long before the script finishes or is interrupted, it becomes impossible to tell when it started failing and therefore where to restart the script from. This change sets a limit on the maximum number of failures to tolerate before exiting the script. Parsing errors are related to the format of the data and don't have implications on the overall success of the script on other data, so no change is implemented for parsing errors.

Summary

Release Note

Documentation

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 48.44%. Comparing base (488eb97) to head (da72164). Report is 143 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2148 +/- ## =========================================== - Coverage 66.46% 48.44% -18.02% =========================================== Files 92 80 -12 Lines 9258 6659 -2599 =========================================== - Hits 6153 3226 -2927 - Misses 2359 3035 +676 + Partials 746 398 -348 ``` | [Flag](https://app.codecov.io/gh/sigstore/rekor/pull/2148/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sigstore) | Coverage Δ | | |---|---|---| | [e2etests](https://app.codecov.io/gh/sigstore/rekor/pull/2148/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sigstore) | `?` | | | [unittests](https://app.codecov.io/gh/sigstore/rekor/pull/2148/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sigstore) | `48.44% <ø> (+0.76%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sigstore#carryforward-flags-in-the-pull-request-comment) to find out more.

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sigstore / rekor