remote.c: report specific errors from branch_get_upstream

remote.c: report specific errors from branch_get_upstream

When the previous commit introduced the branch_get_upstream
helper, there was one call-site that could not be converted:
the one in sha1_name.c, which gives detailed error messages
for each possible failure.

Let's teach the helper to optionally report these specific
errors. This lets us convert another callsite, and means we
can use the helper in other locations that want to give the
same error messages.

The logic and error messages come straight from sha1_name.c,
with the exception that we start each error with a lowercase
letter, as is our usual style (note that a few tests need
updated as a result).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>