GitLab doesn't have a suitable API for accessing diff contents, so our

only option is to access a non-API URL for fetching diff contents. While

this used to work with private repositories via a Personal Access Token,

it hasn't worked for about a year now, as they've removed that

functionality from all public URLs to address a security issue.
This does still work on older versions or for public repositories,

though, so we want to keep the capability around. To do this, we're now

explicitly checking for a HTTP 404, and if we find it, we provide a

useful blurb of text explaining the situation and linking to a KB

article for more details.
As part of this, review request creation and updating now handles

HostingServiceError responses gracefully.

Unit tests pass.
Manually tested this error condition and saw a useful error message.
Manually tested against a public repository and saw the commit go

through.