Add ErrorHandler interface, as well as ErrorHandler\Throwing
and ErrorHandler\Collecting. The error handler is passed to
Parser::parse(). This supersedes the throwOnError option.
NameResolver now accepts an ErrorHandler in the ctor.
Adding this as an option to avoid breaking people's tests.
Some of the test results show pretty clearly that we are incorrectly
assigning the same comment multiple times for nested nodes (mentioned
in #36).
