Show error message on invalid keybindings

[kuznetsovvj]

A copy of the draft from the project to link the merge request.

For example, the following keybinding will silently throw the error Requested value '<CHARACTER>' was not found.

keybindings:
  - command: "focus left"
    binding: "Alt+;"

When validating the user config (ie. ValidateDeserializedConfig method in EvaluateUserConfigHandler.cs), check whether the binding can be cast to Keys enum and throw FatalUserException if it's invalid.

[kuznetsovvj]

I created pull request #157

[kuznetsovvj]

Done