[build] Output a message when can't find log file

When the build tries to start an elasticsearch instance but the start fails
if it fails to find the log file then we log a line about how we can't find
the file.

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

@@ -275,6 +275,8 @@ class ClusterFormationTasks {
                 File logFile = new File(home, "logs/${clusterName}.log")
                 if (logFile.exists()) {
                     logFile.eachLine { line -> logger.error(line) }
+                } else {
+                    logger.error("Couldn't start elasticsearch and couldn't find ${logFile}")
                 }
                 throw new GradleException('Failed to start elasticsearch')
             }