When ExecutionSignature references a number that is out of bound provide a better error message.

For example given the following:

  typedef void ControlSignature(FieldIn<> position,
                                FieldIn<> value,
                                FieldOut<> clusterIdx,
                                FieldOut<> correction,
                                WholeArrayIn<> centers,
                                WholeArrayIn<> fnConsts,
                                WholeArrayIn<> fnSlopes);
  typedef void ExecutionSignature(_1, _2, _3, _4, _5, _6, _7, _8, _9);

The compile error that _8 and _9 generate a compile errors in brigand::at instead of doing a compile time assert with a nice message.