Check that a label is valid before waiting on it

src/main/java/org/jenkins/plugins/lockableresources/LockStepResource.java

@@ -78,11 +78,15 @@ public void validate() throws Exception {
 
 	/**
 	 * Label and resource are mutual exclusive.
+	 * The label, if provided, must be configured (at least one resource must have this label).
 	 */
 	public static void validate(String resource, String label, int quantity) throws Exception {
 		if (label != null && !label.isEmpty() && resource !=  null && !resource.isEmpty()) {
 			throw new IllegalArgumentException("Label and resource name cannot be specified simultaneously.");
 		}
+		if (label != null && !LockableResourcesManager.get().isValidLabel( label ) ) {
+			throw new IllegalArgumentException("The label does not exist: " + label);
+		}
 	}
 
 	private static final long serialVersionUID = 1L;
@@ -108,6 +112,9 @@ public static FormValidation doCheckLabel(@QueryParameter String value, @QueryPa
 			if ((resourceLabel == null) && (resourceName == null)) {
 				return FormValidation.error("Either label or resource name must be specified.");
 			}
+			if (resourceLabel != null && !LockableResourcesManager.get().isValidLabel(resourceLabel)) {
+				return FormValidation.error("The label does not exist: " + resourceLabel);
+			}
 			return FormValidation.ok();
 		}