Constant.__repr__ should be updated to say 'uncertainty' not 'error' and 'unit' not 'units' #4145

As highlighted in this StackOverflow question, it's a bit confusing that the uncertainty on constants is referred to as 'uncertainty' when accessed as an attribute but 'error' in the repr (similarly 'units' in repr vs 'unit' as attribute). We should update __repr__ to show unit= and uncertainty=.