Synonyms autocomplete field does not fail if unknown value entered

Problem/Motivation

When using the synonyms-enabled autocomplete field, if a tag is typed in and not matched via the dropdown list it assigns the first tag found alphabetically in the list, instead of failing with a message indicating an unknown item was entered.

Steps to reproduce

Configure a tag field to use the synonyms autocomplete field.
Make sure the "Create referenced entities if they don't already exist" option is enabled on the field.
Create a piece of content that shows the autocomplete field.
In the field, fill in a value that doesn't exist in the source, and don't select an item in the autocomplete field.
Click "Save".

What happens:
* The first possible match in the source will be added to the field.

What should happen:
* An error is shown indicating that an unknown value was entered in the field.

Proposed resolution

Fix the autocomplete field so it does not save unwanted values.

Remaining tasks

Fix the bug =)

User interface changes

The synonyms-autocomplete widget works

API changes

TBD

Data model changes

n/a