Minor improvements in translation messages in sonata_type_model_autocomplete

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

Added info about form option `multiple` in the error messages to help users understand the error, refs #1764