rebase -i: clean error message for --continue after failed exec

After an "exec false" stops the rebase and gives the control back to
the user, if changes are added to the index, "rebase --continue" fails
with this message, which may technically be correct, but does not point
at the real problem:

.../git-rebase--interactive: line 774: .../.git/rebase-merge/author-script: No such file or directory

We could try auto-amending HEAD, but this goes against the logic of
.git/rebase-merge/author-script (see also the testcase 'auto-amend only
edited commits after "edit"' in t3404-rebase-interactive.sh) to
auto-amend something the user hasn't explicitely asked to edit.

Instead of doing anything automatically, detect the situation and give a
clean error message. While we're there, also clarify the error message in
case '. "$author_script"' fails, which now corresponds to really weird
senario where the author script exists but can't be read.

Test-case-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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