identify unresolvable Mapped types

identify unresolvable Mapped types

Fixed issue where use of an unknown datatype within a :class:`.Mapped`
annotation for a column-based attribute would silently fail to map the
attribute, rather than reporting an exception; an informative exception
message is now raised.

tighten up iteration of names on mapped classes to more fully
exclude a large number of underscored names, so that we can avoid trying
to look at annotations for them or anything else.  centralize the
"list of names we care about" more fully within _cls_attr_resolver
and base it on underscore conventions we should usually ignore,
with the exception of the few underscore names we want to see.

Fixes: #8888
Change-Id: I3c0a1666579fe67b3c40cc74fa443b6f1de354ce