TPAC: Invalid due dates cause 500 server error

TPAC: Invalid due dates cause 500 server error

CStore appears to return dates with leading 0s (such as '0212-10-08
23:59:59-05:17:32') with the leading 0s stripped off, resulting in
cases with 1-digit or 3-digit years for which DateTime::Format::ISO8601
returns an error.

We can protect against this problem by adding some defensive code to the
TPAC utility method to add the 0s back to the start of the year. We can
also log the problem when it occurs so that administrators can fix the
problem dates in the database.

Signed-off-by: Dan Scott <dan@coffeecode.net>