Abort early in watch multi commands

During refactoring of Continuous, I inadvertently changed the semantics
of `~` so that all multi commands were run regardless of whether or not
an earlier command had failed. I fixed the issue and added a regression
test.

main/src/main/scala/sbt/internal/Continuous.scala

@@ -243,7 +243,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
               case exec :: rest =>
                 val updatedState = MainLoop.processCommand(exec, s.copy(remainingCommands = rest))
                 val remaining =
-                  updatedState.remainingCommands.takeWhile(_.commandLine != FailureWall)
+                  updatedState.remainingCommands.takeWhile(_.commandLine != failureCommandName)
                 remaining match {
                   case Nil =>
                     updatedState.remainingCommands.forall(_.commandLine != failureCommandName)
@@ -351,7 +351,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
               currentCount.getAndIncrement()
               callbacks.beforeCommand()
               // abort as soon as one of the tasks fails
-              valid.takeWhile(_._3.apply())
+              valid.takeWhile { case (_, _, task) => task() }
               updateLegacyWatchState(s, configs.flatMap(_.inputs().map(_.glob)), currentCount.get())
               ()
             }

sbt/src/sbt-test/watch/multi-error/build.sbt

@@ -0,0 +1,8 @@
+import java.nio.file._
+watchOnIteration := { (_, _, _) => Watch.CancelWatch }
+
+val copyFile = taskKey[Unit]("copy a file")
+copyFile := {
+  val base = baseDirectory.value.toPath
+  Files.copy(base / "changes" / "Foo.txt", base / "Foo.txt")
+}

sbt/src/sbt-test/watch/multi-error/changes/Foo.txt

@@ -0,0 +1 @@
+foo

sbt/src/sbt-test/watch/multi-error/src/main/scala/Foo.scala

@@ -0,0 +1 @@
+object Foo {

sbt/src/sbt-test/watch/multi-error/test

@@ -0,0 +1,3 @@
+> ~compile; copyFile
+
+$ absent Foo.txt