Panzer: add better error handling for teko coordinate callback interface
Created by: rppawlo
When the teko callback interface is used to register coordinate support, if the "Coordinates" field name can't be found in the DOF manager it assumes that it must be in the auxiliary DOF Manager and just accesses the object. If the aux_blocked_ugi_ is null then we get a seg fault. Additionally even if it exists, we need to add a check to make sure the field exists in the aux_blocked_ugi_ object.
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