I think the decimal exceptions should continue to be raised as-is. 
There is a well-defined and thought-out structure for the Decimal
exceptions.  Also, the constructor's exceptions tend to be raised in an
environment where there are other decimal operations taking place -- it
would be a shame to have to catch both types of exception.

I spent about a month working on this module and put a decent amount of
thought into integrating the API.  I would prefer to not revisit all of
those decisions one at a time.

Instead, let's graft on any new magic methods and fix straight-out bugs.  

Having lots of little microscopic API changes will harm more than it
will help.  Why introduce incompatabilities for no gain.

I'm inclined to dismiss this whole bug report.  It doesn't help anything
to muck with the existing choices about whitespace handling.  Recommend
closing this as yagni / don't-care.