Batch contribution entry may drop contributions without indication to user

Steps to reproduce:

Set up an existing donation to conflict with.

• Log into demo site
• Add a contribution to a contact
• Amount=$1, Type=Donation, expand additional info, Invoice ID=111-111
• Save

Test that batch contributions work OK.

• Contributions > Batch Data Entry => New Data Entry Batch
• Number of items=4, Total amount=4
• Fill in details for multiple contributions but without conflicting Invoice ID. (I used Invoice ID 111-112 to 111-115.)
• Batch data entry screen gives no feedback on submission (CRM-19573), but you can review contact donations to confirm (I used demo@example.com)

Test that batch contributions do not work if conflicting ID submitted

• Repeat process as above, but use the Invoice ID from first contribution (ex 111-111) as the second row of the contributions. (I used Invoice IDs 111-116, 111-111, 111-117, 111-118.)
• There is an E_NOTICE Notice: Undefined property: CRM_Core_Error::$id in CRM_Batch_Form_Entry->processContribution() (line 607 of /srv/buildkit/build/dmaster/sites/all/modules/civicrm/CRM/Batch/Form/Entry.php) which will not be visible on most sites
• There is no feedback on the success or failure of the batch data input (CRM-19573)
• Contribution entries which failed due to duplicate Invoice ID are silently dropped

The outcome of the above test is that the second attempt at creating a contribution which has a pre-existing Invoice ID will fail silently.

The second batch will be recorded as containing 4 * $1 entries for a total of $4, but only 3 * $1 entries are created, so the accounting will be mismatched.