Unify all error handling for the server

[vinistock]

We cannot use blanket rescue statements for the entire server because the behaviour for handling errors is different between requests and notifications.

For requests, we want to return error: message instead of the typicaly result: something. And for notifications, we cannot return anything, we can only log that an error occurred.

The approach that I took here is to create handlers that we can use both ourselves and in server add-ons, that provide appropriate error handling for requests or notifications.

I also created convenience methods for returning responses or errors.

[vinistock]

• Unify all error handling for the server #527 👈 (View in Graphite)
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[vinistock]

We need to ensure that the dummy files are indexed to be able to test our own features. A recent optimization in the LSP made us fully ignore the entire test directory straight away.