Raise on flag_modified() for non-present attribute

Raise on flag_modified() for non-present attribute

The :func:`.attributes.flag_modified` function now raises
:class:`.InvalidRequestError` if the named attribute key is not
present within the object, as this is assumed to be present
in the flush process.  To mark an object "dirty" for a flush
without referring to any specific attribute, the
:func:`.attributes.flag_dirty` function may be used.

Change-Id: I6c64e4d253c239e38632f38c27bb16e68fe8dfbe
Fixes: #3753