t/t5505-remote: test failure of set-head

t/t5505-remote: test failure of set-head

The test coverage was missing a test for the failure branch of remote
set-head auto's output. Add the missing text and while we are at it,
correct a small grammatical mistake in the error's output ("setup" is
the noun, "set up" is the verb).

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>