Acq: When building invoices from EDI messages, avoid bad data

From some vendors, these EDI messages contain strings (useless ones,
like just the name of the vendor) where we had been expecting numeric
identifiers.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>
Acq: When processing EDI invoices, skip unknown line item references

See LP #1172373.

In their electronic invoices, vendors sometimes include a mix of line
items that your ILS knows about, because you ordered them through it,
and line items of which your ILS knows nothing. We should not fail
altogether at processing invoices, but instead process what line items
we can.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>