fix: fail when no style is explicitly configured (don't use the default style)

[andreoliwa]

Issues fixed by this pull request: Fix #491

Proposed changes

1. Fail when no style is explicitly configured.
2. Suggest to run nitpick init or do it manually.

Checklist

• [x] Read the contribution guidelines
• [x] Run make locally before pushing commits
• [x] Add tests for the relevant parts:
  • [x] API
  • [x] CLI
  • [x] flake8 plugin (normal mode)
  • [x] flake8 plugin (offline mode)
• [x] Write documentation when there's a new API or functionality

[coveralls]

Pull Request Test Coverage Report for Build 5912450428

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 96.329%

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Totals
Change from base Build 5909458222:	0.003%
Covered Lines:	1994
Relevant Lines:	2053

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💛 - Coveralls

[github-actions[bot]]

:tada: This PR is included in version 0.35.0 :tada:

The release is available on:

• v0.35.0
• GitHub release

Your semantic-release bot :package::rocket: