Introduce APIError type and make ErrorEvent.Err public

[alexbakker]

This makes it a little bit easier to act on API errors that happen while streaming.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08 :tada:

Comparison is base (9faaa4f) 87.51% compared to head (eb423d3) 87.60%.

:exclamation: Current head eb423d3 differs from pull request most recent head 972ffb4. Consider uploading reports for the commit 972ffb4 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #177 +/- ## ========================================== + Coverage 87.51% 87.60% +0.08% ========================================== Files 15 15 Lines 1354 1363 +9 ========================================== + Hits 1185 1194 +9 Misses 125 125 Partials 44 44 ``` | [Impacted Files](https://codecov.io/gh/mattn/go-mastodon/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn) | Coverage Δ | | |---|---|---| | [helper.go](https://codecov.io/gh/mattn/go-mastodon/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn#diff-aGVscGVyLmdv) | `100.00% <100.00%> (ø)` | | | [streaming.go](https://codecov.io/gh/mattn/go-mastodon/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn#diff-c3RyZWFtaW5nLmdv) | `97.58% <100.00%> (ø)` | | | [streaming\_ws.go](https://codecov.io/gh/mattn/go-mastodon/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn#diff-c3RyZWFtaW5nX3dzLmdv) | `96.15% <100.00%> (ø)` | | 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=mattn). 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=mattn)

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[mattn]

Thank you