INT-2864 Enforce Ref/Expression Mutual Exclusivity

Delegating consumer endpoints (`<service-activator/>` etc)
may not have both 'expression' and 'ref' attributes.
Previously, 'expression' was ignored when 'ref' was present.

- Enforce mutual exclusivity in AbstractDelegatingEndpointParser.
- Add tests for illegal combinations.
- Improve error message when 'method' used with `<expression/>`.
- Change parser to use reader.error() instead of Assert.
- Add element description to all parser errors.