Revert "Build: Fail integ test if elasticsearch fails to start on windows"

This reverts commit f974d5e470.

Apprently spawn does not support failonerror in ant.

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy

@@ -139,7 +139,7 @@ class ClusterFormationTasks {
             // elasticsearch.bat is spawned as it has no daemon mode
             start = project.tasks.create(name: "${task.name}#start", type: DefaultTask, dependsOn: setup) << {
                 // Fall back to Ant exec task as Gradle Exec task does not support spawning yet
-                ant.exec(executable: 'cmd', spawn: true, dir: home, failonerror: true) {
+                ant.exec(executable: 'cmd', spawn: true, dir: home) {
                     esEnv.each { key, value -> env(key: key, value: value) }
                     (['/C', 'call', 'bin/elasticsearch'] + esArgs).each { arg(value: it) }
                 }