Handle queries with unmapped values

[idimopoulos]

In #7 a workaround was added in order to allow the field storage config update the a field storage when there is no mapping yet (usually when the field is first created). This is a temporary workaround and the following need to be implemented instead:

• The workaround should throw an exception instead of throwing an error. There should not be any case of a query running for an unmapped field.
• In rdf_entity_form_field_storage_config_edit_form_alter, a new #validate entry should be unshifted (array_unshift) so that it is thrown before the storage config validator. The new validator should update teh new mapping in the field handler service for all bundles or for the bundles applied to the field if possible.

The above will allow the storage form to work properly and also ensure that no wrong queries are running.

[idimopoulos]

The solution above is to be discussed.