`CqlValue` conversion methods - retain `Self` in `Result::Err()` upon conversion failure

[wprzytula]

Problem

Some CqlValue conversions consume Self upon error, hindering further interaction with such CqlValue.

For example, conversion method into_vec(self) returns Option<Vec<CqlValue>>, returning None in case of conversion failure, but consuming self.

Suggested solution

Return Result<EndType, Self>, this way preserving self in case of conversion failure. This is API-breaking.