Improve Merge Request Error Messaging

Proposal

Currently when there is a timeout on loading the diffs on a large merge request a generic error is surfaced that could be caused by a variety of other issues:

This error can also be seen on other timeouts such as loading a large amount of comments or when there is invalid data in a comment or file preventing the diff from rendering. This is not information that can be determined using the UI. A Self-Managed user will need to consult the logs and attempt to determine the source of the error and a GitLab.com or Dedicates user will need to contact Support for additional assistance.

This request is for an improvement to this error message that surfaces the error and indicates the component generating it.