perl: update our ancient copy of Error.pm

perl: update our ancient copy of Error.pm

The Error.pm shipped with Git as a fallback if there was no Error.pm
on the system was released in April 2006. There's been dozens of
releases since then, the latest at August 7, 2017. Let's update to
that.

I don't know of anything we need from this new release or which this
fixes. This change is simply a matter of keeping up with
upstream. Before this users who'd install git via their package system
would get an up-to-date Error.pm, but if it's installed from source
they'd get one more than a decade old.

This undoes a local hack we'd accumulated in 96bc4de85c ("Eliminate
Scalar::Util usage from private-Error.pm", 2006-07-26), it's been
redundant since my d48b284183 ("perl: bump the required Perl version
to 5.8 from 5.6.[21]", 2010-09-24).

This also undoes 3a51467b94 ("Typo fix: replacing it's -> its",
2013-04-13). This is the Nth time I find that some upstream code of
ours (in contrib/, in sha1dc/ and now in perl/ ...) has diverged from
upstream because of some tree-wide typo fixing. Let's not do those
fixes against upstream projects, it's more valuable that we have a 1=1
mapping to upstream than to fix typos in docs we never even generate
from this code. If someone wants to fix typos in them fine, but they
should do it with a patch to upstream which git.git can then
incorporate.

The upstream code doesn't cleanly pass a --check, so I'm adding a
.gitattributes file for similar reasons as done for sha1dc in
5d184f468e ("sha1dc: ignore indent-with-non-tab whitespace
violations", 2017-06-06).

The updated source was retrieved from
https://fastapi.metacpan.org/source/SHLOMIF/Error-0.17025/lib/Error.pm

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>