[IMP] orm: make error handling more consistent when accessing deleted/filtered records

[IMP] orm: make error handling more consistent when accessing deleted/filtered records

Previous behavior was unspecified and untested - leading to random results
when performing operations on a mix of deleted and ir.rule-filtered records.

The behavior is now clarified and explicitly tested.
One suprising case remains: read() on a deleted record
returns an empty result instead of raising an error,
in order to avoid spurious errors when a client
performs a sequence of search(), read() while
another user is deleting records.

bzr revid: odo@openerp.com-20121109171451-z2m6oqs910103lcz