DrillOptiq should raise appropriate error message while dealing with unknown RexNode

Currently for certain types of RexNodes (RexOver, RexCorrelVariable) DrillOptiq does not convert it to the equivalent logical expressions. In that case we simply return a NullExpression (minor type: Null) and we error out later in execution when we try to allocate a vector with minor type Null. We should error out early in DrillOptiq that there was a planning issue indicating the particular RexNode that wasn't handled correctly.