Issue error if vtkAlgorithm::GetInputConnection called on wrong port

The original logic was issuing a warning if querying a connection that was outside the range of input connections to the port. This led to warnings in application code that was querying if a particular algorithm had input connections yet.

The new logic issues an error if the port in reference, is outside the normal range of input ports for the algorithm but silently returns a nullptr when the connection index is outside the range of connection indices for the port i.e. no more warning when no input is provided.

Backport: release