LP#1709932: recognize more strings from oils_i18n_gettext()

LP#1709932: recognize more strings from oils_i18n_gettext()

This patch makes db-seed-i18n.py more flexible about
where commas between strings in invocations of oils_i18n_gettext()
can appear, reduces the chances that strings fail to be grabbed
for translation. It also improves the error message shown
when parsing errors occur.

To test
-------
[1] Apply patch and run

    cd build/i18n && make sql2pot

[2] Verify that no error messages are produced during
    this step.
[3] Inspect the changes to po/db.seed/db.seed.pot and
    verify that additional strings are added, including
    "Item Print Label Font Family".
[4] (optional) Deliberately remove a comma from an
    oils_i18n_gettext invocation in 950.data.seed-values.sql
    and run make sql2pot again. The error message shoudl
    now include the failing chunk of text.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>