EntityAutocomplete form element allows multiple values when #tags is FALSE

When you have an entity_autocomplete form element (not talking about field widget here) with '#tags' => FALSE, then it should only accept a single value. There are actually two issues here, but I'm combining them because they're so closely related:

Problem

1. The autocomplete works the same as if #tags were TRUE. So if you type in a comma and another letter, it's going to autocomplete that, which suggests to the user that you can enter multiple items.
2. When multiple values are actually supplied, the form element takes it upon itself to choose one of them as the correct value and drops the rest. (It chooses the last one entered.) However, there's no way that the form element can know which of the values is actually the one that should be used.

Proposed solution

1. Do not autocomplete past the first item.
2. Add a form error if multiple values were entered.