Add `ValidationException`, similar to `SafeException`

When a reactive element depends on an input or a specific range of input values, app authors have limited options:

1. Use req(input.foo()) to prevent the reactive from running, but this happens silently.
2. Raise a SafeException describing the problem, but this renders in the client as a red Error: message.
3. Use a custom UI element as a replacement.

    @output
    @render.plot
    def barchart():
        if not len(input.traits()):
            raise SafeException("Please select one or more traits.")

        if not len(input.breeds()):
            raise SafeException("Please select one or more breeds.")

Instead of this process, it would be helpfup if authors could raise a ValidationError that doesn't render as an error. Maybe something like this:

    @output
    @render.plot
    def barchart():
        if not len(input.traits()):
            raise ValidationException("Please select one or more traits.")

        if not len(input.breeds()):
            raise ValidationException("Please select one or more breeds.")

Here's an example app that motivated this issue

Start by tracing the existing SafeException handling from the issue's reactive output example and compare how its message is rendered in the client. Determine the relevant reactive and output-rendering entry points, then define tests for a ValidationException whose message is shown without the red Error treatment. Done means the new exception behavior supports the input-validation examples in the issue.