More C++ while loop validation

With this change, we call IsValidOutputTensor() on the returned
outputs from the condition and body functions. This will return a bad
status if no output or a null output is set, or if an output has a bad
index.

This also adds unit tests for related error cases to the C and C++
unit tests. They often produce different errors because the C
implementation goes through the graph constructor.

PiperOrigin-RevId: 167925641