Consistent messaging about SCT range.

[filipsch]

In has_equal_value(), has_equal_formula() and similar functions, check_range() is the first thing that happens to see if the value/formula exists in the range specified in test_exercise(), and then zoom in. If it fails, it throws a meaningful message. If it passes, it just zooms in.

In line with how other whats work, sheetwhat could gradually build a list of messages, the first one of which can be "Check cell(s) X.". Next, other functions can then append messages to the list that don't have to worry anymore about mentioning the SCT range. This makes the functions more self-contained.