GetColumnValue: check if len == SQL_NULL_DATA instead of ret
GetColumnValue: add support for returning really long string column values
GetSQLError: re-introduce the explicit message property
test: udpate some tests
test: reduce number of iterations; causing false failure due to timeout
GetColumnValue: remove comparison of len < 0

That was just an alternate way of checking len == SQL_NULL_DATA
test: add note to test
GetColumnValue: use handle scope
0.5.23