R1:87e9c936ad1c

Make "git cat-file" exception messages include repository monogram/slug

Summary:
When throwing an exception related to output provided by `git cat-file`, include the repository monogram to allow potentially debugging in Git.

Closes T15661

Test Plan:
Unclear. Basically: "have a broken Git repository in Diffusion".
(However this patch changes a message only shown in case of an exception, so in the worst case we'd break the exception via an exception.)

Reviewers: O1 Blessed Committers, speck

Reviewed By…