Display an error message if the RuboCop config is invalid

[vinistock]

Motivation

Closes #225

If a project using the LSP has invalid RuboCop configuration, like using a cop that no longer exists, diagnostics and formatting break. While this is not an error of the LSP itself, it would be nice to alert users and prompt them to fix their configuration.

Implementation

Rescue RuboCop's validation error when trying to run diagnostics and send a show message notification. This makes VS Code display an error notification if the configuration is invalid.

Automated Tests

Added a new integration example.

Manual Tests

1. Start the LSP on this branch
2. Change .rubocop.yml to make it invalid. For example, add

InvalidCop:
  Enabled: true

3. Verify you get a notification about the invalid configuration

[Morriar]

I wonder if the message should point fingers, something like: "There is an error with your Rubocop configure. Check your .rubocop.yml file."

So people understand the issue is not with ruby-lsp?

[vinistock]

@Morriar I changed the phrasing to make it a bit more explicit.