Clear Diagnostics On Unacceptable Exceptions

[ObliviousHarmony]

All Submissions:

• Have you checked for duplicate PRs?
• Have you added an entry to the CHANGELOG.md file's [Unreleased] section?

Changes proposed in this Pull Request:

We should be clearing diagnostic information when we throw
exceptions that we did not accept. This prevents some
diagnostics to be left behind when they shouldn't be.

Closes #71.

How to test the changes in this Pull Request:

1. Open a project with a working PHPCS extension configuration.
2. Make sure that there are visible errors in a document.
3. Break the configuration, such as by turning off autoExecutable and entering an invalid executable.