Return nicer error if trying to pull from non-existent user (#18288)
Return nicer error if trying to pull from non-existent user (#18288)

* Return nicer error if trying to pull from non-existent user

Gitea serv will currently return an 500 if we try to pull from a repository where
the owner does not exist.

This PR checks for the UserNotExist Error when checking for the user and will
return a NotFound error instead.

Fix #18225
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea