Soft credit "contact" field validation errors do not display

To reproduce, case 1:

1. Create a new contribution (or modify an existing one). Open this contribution for editing.
2. Select a contact in the "Contact" field under the "Soft Credit" section. Save the contribution. Observe that it is saved correctly.
3. Again open the contribution for editing. Under the "Soft Credit" section, click "another soft credit" and again select the same contact in the "Contact" field; you'll now have two rows under the "Soft Credit" section, both attributed to the same contact.
4. Save the contribution. Observe that the "Edit Contribution" form reloads without saving, as if it had failed validation, and that no validation error is displayed.

To reproduce, case 2:

1. Create a contribution page (or modify an existing one) and set "Honoree Section Enabled" = yes; and select a profile in the "Honoree Profile" field.
2. Create a new contribution (or modify an existing one), selecting the contribution page  from Step 1 in the "Online Contribution Page" field. Save the contribution.
3. Edit the contribution from Step 2, selecting an individual contact in the "Contact" field under the "Soft Credit" section. Save the contribution. Observe that it is saved correctly.
4. Edit the contribution again, this time selecting an organization contact in the "Contact" field under the "Soft Credit" section. Save the contribution. Observe that the "Edit Contribution" form reloads without saving, as if it had failed validation, and that no validation error is displayed.

Apparent cause:

• The error messages for these two validation rules are attributed to fields named `soft_credit_contact[$id]`, whereas the actual name of the relevant field is `soft_credit_contact_id[$id]`.  See:

• • https://github.com/civicrm/civicrm-core/blob/4.7.18/CRM/Contribute/Form/SoftCredit.php#L235
  • https://github.com/civicrm/civicrm-core/blob/4.7.18/CRM/Contribute/Form/SoftCredit.php#L247