am: improve author-script error reporting

am: improve author-script error reporting

If there are errors in a user edited author-script there was no
indication of what was wrong. This commit adds some specific error messages
depending on the problem. It also relaxes the requirement that the
variables appear in a specific order in the file to match the behavior
of 'rebase --interactive'.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>