Change JsonNode.asXxx() methods with no defaults to throw exception; add in TreeNode

[cowtowncoder]

Currently various asXxx() methods (like, say, asInt()) return default value (like 0 for int) in case where no conversion is possible. But it would usually be better to instead throw an exception since there are already variants that allow passing of default value to return in invalid case.
Throwing exception instead would allow caller to avoid potentially odd and unhelpful coercions into default value. Given existence of alternatives with defaults no functionality would be removed.

In addition to this change it would make sense to add (demote) these methods in TreeNode, esp. since we can use Java 8 default methods for interfaces.