bool(Enum) should raise ValueError

[Erik Aronesty]

I just spend a while tracking down and killing all "if Enum" and "if not
Enum" bugs in my code.   I was frankly shocked that this didn't raise a
ValueError to begin with.

Apparently all enums are true/false depending on whether the underlying
value is truthy or falsy.

Which breaks the abstraction Enum's are trying to achieve because now the
user of an Enum has to know "stuff" about the underlying value and how it
behaves.