fix: makes exceptions public so others can import and handle them

[julianSelser]

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Fixes #1122 #1123 ☕️

Makes exception public for clients of the library to import and handle them.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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[TimurSadykov]

Thanks for contributing! Exceptions are package private by design, but you can catch them using interfaces. More details in the issue.

Feel free to change this PR into OAuth2Error implementation.