Add ability to abort the processing of outputs

abortOutput() leaves the state of the output unchanged,
unlike req(), validate(), or stop().

man/abortOutput.Rd

@@ -0,0 +1,20 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/utils.R
+\name{abortOutput}
+\alias{abortOutput}
+\title{Abort processing the current output}
+\usage{
+abortOutput()
+}
+\description{
+Signals an error that Shiny treats specially if an output is currently being
+evaluated. Execution will stop, but rather than clearing the output (as
+\code{\link{req}} does) or showing an error message (as \code{\link{stop}}
+does), the output simply remains unchanged.
+}
+\details{
+If \code{abortOutput} is called in any non-output context (like in an
+\code{\link{observe}} or \code{\link{observeEvent}}), the effect is the same
+as \code{\link{req}(FALSE)}.
+}
+