Improve error message when the diff in Pull Request is exceeding UI viewing limits

When a pull request diff is not loaded in the UI because it exceeds the Limits for viewing content and diffs, a generic error message is presented ("Something went wrong") with a button to "Try again", which in most cases will not work due to the size of Pull Request.

It would be good to improve the error message informing the user that the pull request is too big to be loaded in the UI, and provide instructions to check the diff locally.