Expose error "This alias has already been taken" for Slack integration

Proposal

When a user tries to setup Slack integration on a new project and another project was already setup with that alias name, the integration fails and the UI shows 422 error - 422: The change you requested was rejected

This is not clear to the user why the failure occured.

Backend Rails will show the exception as:

"exception.message": "Validation failed: Alias This alias has already been taken"

This gives a better idea that the alias name should be changed. We should make this clear in place of the 422 error.