Improve startup failures user experience

- Be clear about config errors instead of difficult stack traces
- Fix default values which should not cause startup failures

src/main/groovy/io/kamax/mxisd/exception/ConfigurationException.java

@@ -0,0 +1,11 @@
+package io.kamax.mxisd.exception;
+
+public class ConfigurationException extends RuntimeException {
+
+    private String key;
+
+    public ConfigurationException(String key) {
+        super("Invalid or empty value for configuration key " + key);
+    }
+
+}