Check for errors on object through valueBinding

Currently, `errorView` looks for errors on the object using the `label` value. This might not always work, as the `label` value might not correspond to the object's field name. For example the following code looks for errors on the field with name `IBAN`, but it should be looking for `iban`. Currently the only way to do this is not providing a custom label, but ideally I want to have a humanized label value that is different from the field's name.

    {{view Bootstrap.Forms.TextField valueBinding="iban" label="IBAN"}}

This PR is backwards compatible when not using a `valueBinding`. If a `valueBinding` is present, it will use that field to look for errors. I've also added a test for the new use case.