Add Custom Messages to Errors

[rcosta358]

This PR adds support for custom messages in refinements and state refinements, through an extra parameter msg:

@Refinement(value="...", msg="...")
@StateRefinement(from="...", to="...", msg="...")

Examples

In the last example, the close method is annotated with two state refinements, each with its own message, so both are shown. However, to simplify the error message, users should specify a message in only one of them, e.g., "The buffered reader should be open or marked to be closed".