A first attempt of the custom error type for unsatisfied input dependencies

Example:

```s
runApp(list(
  ui = fluidPage(
    checkboxGroupInput('foo', 'Check some letters', choices = head(LETTERS)),
    selectizeInput('bar', 'Select a state', choices = state.name),
    plotOutput('plot')
  ),
  server = function(input, output) {
    output$plot <- renderPlot({
      renderError(
        list(message = 'Check at least one letter!', condition = is.null(input$foo)),
        list(message = 'Please choose a state.', condition = input$bar == '')
      )
      plot(1:10, main = paste(c(input$bar, input$foo), collapse = ', '))
    })
  }
))
```