str() for non-parametrized exceptions still defaults to class docstring
but now it is possible to simply override the message by defining
class.msg variable.

All list entries are converted to strings before the list is printed.
I.e. one-item list is printed as 'vm-123.idm.lab.eng.brq.redhat.com. IN NS'
instead [<DNS vm-123.idm.lab.eng.brq.redhat.com. IN NS RRset>].

This change makes __init__ and __str__ parametrization simpler
and keeps backward compatiblity with old code which does not use keyword
arguments in raise().

Parametrized __str__ can be implemented simply by defining "fmt" and
"supp_kwargs" class variables in particular exception class.

Doc string is used in cases where specific message parameters were not provided
during instantiation/raise.

Doc string is used in cases where more specific message was not provided
during instantiation/raise.