* Fix crash due to mixing legacy filters and expressions
In some cases, (invalid) nested filters that used a mix of legacy filter
syntax and expression syntax caused a crash due to a failure to
propagate parsing errors from deeper within the filter expression.
These errors went undetected in part because these conversion functions
returned unique_ptr<Expression> values (or vectors thereof), using
{nullptr} to represent a parsing error, but the core expression classes
expect unique_ptr<Expression> that are never null.
This changes over to using expression::ParseResult (aka
optional<unique_ptr<Expression>>), to represent conversion failure
the same way we do in the rest of the expression system.
* Fix clang 3.8 / gcc 4.9 issue
2da57bd157