extend methods of decimal module

[Wolfgang]

Uhh, the curse of not copy-pasting everything:

> >>> exp(200000)

should, of course, read
>>> epx(190000)

> Traceback (most recent call last):
>   File "<pyshell#46>", line 1, in <module>
>     epx(190000)
>   File "C:\Python34\dmath_rev.py", line 27, in epx
>     n *= q
> decimal.Overflow: [<class 'decimal.Overflow'>]
>