Support addition of information messages to a validation context

Currently the UIValidationContext API supports only the addition of errors, and thus the display of only error messages. It would be useful to allow warnings to be added to the context, to enable the UI provider to display them as well.

For example, field that accept a class name should preferably be associated with a validator that can warn about the use of the '$' character. This is not an error, but we should warn users in such scenarios.