Cleanup error types

[a1phyr]

Checklist

• n/a I've added tests for all code changes and additions (where applicable)
• n/a I've added a demonstration of the new feature to one or more examples
• I've updated the book to reflect my changes
• n/a Usage of new public items is shown in the API docs

API changes

• Use #[non_exhaustive] for error::Error instead of __NonExhaustive trick. This is a breaking change, and requires a minimal Rust version of 1.40 (previously 1.38). This notably enables to add new error types without it being a breaking change.
• Change std::error::Error implementation of different error types, to use "state of the art" functions instead of deprecated ones.
• Deprecated NoError and use std::convert::Infallible, and re-export the latter as NoError for a smoother transition (maybe we should remove completly). This will also allow a smoother transition to never type when it is stabilized.

[azriel91]

Nice PR ✌️!
Small change + please could you also add a changelog entry? (edit: can copy paste the API changes part of your comment and link to this PR)
Should be good to go after that.

[azriel91]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push