Improve error message when a transport helper was not found
Perviously, the error message was:
git: 'remote-foo' is not a git-command. See 'git --help'.
By not treating the transport helper as a git command, a more suitable
error is reported:
fatal: Unable to find remote helper for 'foo'
Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi> Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>