Create more understandable OPENWHISK_HOME missing msg (#3)

diff --git a/settings.gradle b/settings.gradle
index 2069ac2..47819fb 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -1,6 +1,11 @@
 def owPath = System.getenv("OPENWHISK_HOME") ?: '../open'
 def owDirectory = new File(owPath)
 
+if (!owDirectory.exists()) {
+    throw new GradleScriptException("Environment variable OPENWHISK_HOME must point to a valid OpenWhisk build", null)
+}
+
+
 include 'common:scala'; project(':common:scala').projectDir = new File(owDirectory, 'common/scala')
 include 'core:controller'; project(':core:controller').projectDir = new File(owDirectory, 'core/controller')
 include 'core:invoker'; project(':core:invoker').projectDir = new File(owDirectory, 'core/invoker')