More descriptive error for non-mapped string prop name

More descriptive error for non-mapped string prop name

Fixed issue where using a loader option against a string attribute name
that is not actually a mapped attribute, such as a plain Python descriptor,
would raise an uninformative AttributeError;  a descriptive error is now
raised.

Fixes: #4589
Closes: #4594
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4594
Pull-request-sha: 2b7ed5240f49be90f9390e3d041c9cb957083465

Change-Id: I66b9937991eb7cdbe074a92f490af1c80d16449e
(cherry picked from commit 7700005a1f3504e26633a55db5cddeac114cb78f)