Consider generalizing Errors so that it also works for more constrained throwers

Unless I missed something, `Errors` currently is used to wrap all throwables. It would be useful to be able to use `Errors` (or a generalized version of it, say, `SpecificErrors`) to wrap interfaces which throw only one checked exception, for example, `IOException`. Thus, `SpecificErrors` would be a generic type, with a type parameter `` which designates the kind of throwable to be wrapped. Any other unchecked exception thrown by the wrappers created by `SpecificErrors` would be left alone and thrown as-is.