util-app: Add visibility for NonFatal exceptions during exiting of an…

… App

Summary: Problem

When exceptions occur in executing `c.t.app.App#close` they are mostly
swallowed -- the `closeOnExitLast` returns the first encountered exception.
However, there is no visibilty on any NonFatal exception from executing
registered `onExit` functions or closing `closeOnExit` Closables, nor over all
NonFatals from closing `closeOnExitLast` Closables.

Solution

Update the logic in `c.t.app.App#close` to allow for logging and collecting
NonFatal exceptions which occur during closing of the app. The collected
exceptions are finally thrown in the new CloseException.

Result

Users will get visibility into NonFatal exceptions which happen during closing
of the application through logging and collection of causes in a thrown
CloseException.

JIRA Issues: CSL-6040

Differential Revision: https://phabricator.twitter.biz/D146029

server/src/test/scala/com/twitter/server/AdminHttpServerTest.scala

@@ -149,6 +149,7 @@ class AdminHttpServerTest extends FunSuite with Eventually with IntegrationPatie
         override protected lazy val shutdownTimer = new MockTimer
 
         override protected def exitOnError(reason: String): Unit = ()
+        override protected def exitOnError(throwable: Throwable): Unit = ()
 
         override def main(): Unit = {
           val drainingClosable = Closable.make { _ =>