If GitHub rate limit is exceeded, dump rate limit info or advise user to set GITHUB_TOKEN

[jwodder]

Closes #137.

[yarikoptic]

looks good to me -- merge if/when CI green

[codecov-commenter]

Codecov Report

Base: 64.30% // Head: 63.61% // Decreases project coverage by -0.68% :warning:

Coverage data is based on head (3bf79d8) compared to base (329a874). Patch coverage: 22.72% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #144 +/- ## ========================================== - Coverage 64.30% 63.61% -0.69% ========================================== Files 1 1 Lines 1423 1443 +20 Branches 278 281 +3 ========================================== + Hits 915 918 +3 - Misses 414 431 +17 Partials 94 94 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-installer/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [src/datalad\_installer.py](https://codecov.io/gh/datalad/datalad-installer/pull/144/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-c3JjL2RhdGFsYWRfaW5zdGFsbGVyLnB5) | `63.61% <22.72%> (-0.69%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad)

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