branch: improve error log on branch not found by checking remotes refs

branch: improve error log on branch not found by checking remotes refs

New git users may want to locally delete remote-tracking branches but
don't really understand how they are distinguished from branches by git.
Then one may naively try:
`git branch -d foo/bar` and get a correct error `branch foo/bar not
found` but hard to understand for a newbie, this patch aims to guide one
in such case.

when failing to delete a branch with `git branch -d <branch>` because
of branch not found, try to find a **remote refs** matching `<branch>`
and if so, add an hint:
`Did you forget --remote?` to the error message

Signed-off-by: Clement Mabileau <mabileau.clement@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>