Replace ConstraintError with ValueError (on assigning None to required attribute) and TypeError (on assigning raw primary key with invalid number of column)
Validation bug fixed
Improved exception message when TypeError is occurred during validation of relationship attribute

Conflicts:
	pony/orm/tests/test_crud.py
Fixes #74: Wrong FK column type when using `sql_type` on foreign ID column
Merge branch 'master' to 'orm': bugfixes