[HER-1687] H3 web UI: adding non-existent job directory gives unhelpful NPE

* Engine.java
    return 'false' rather than NPE when given directory non-existent/contains no .cxml
* EngineResource.java
    better handle empty 'add' path; show NACK flash when 'add' has no effect

engine/src/main/java/org/archive/crawler/framework/Engine.java

@@ -88,10 +88,15 @@ public class Engine {
     }
 
     public boolean considerAsJobDirectory(File dir) {
-        for (File cxml : dir.listFiles(new FilenameFilter() {
+        File[] candidateConfigs = dir.listFiles(new FilenameFilter() {
             public boolean accept(File dir, String name) {
                 return name.endsWith(".cxml");
-            }})) {
+            }});
+        if(candidateConfigs==null) {
+            // directory did not exist or did not contain cxml
+            return false; 
+        }
+        for (File cxml : candidateConfigs) {
             try {
                 CrawlJob cj = new CrawlJob(cxml);
                 if(!jobConfigs.containsKey(cj.getShortName())) {

engine/src/main/java/org/archive/crawler/restlet/EngineResource.java

@@ -26,6 +26,7 @@ import java.io.Writer;
 import java.util.ArrayList;
 import java.util.Collections;
 
+import org.apache.commons.lang.StringUtils;
 import org.archive.crawler.framework.CrawlJob;
 import org.archive.crawler.framework.Engine;
 import org.restlet.Context;
@@ -73,7 +74,17 @@ public class EngineResource extends Resource {
         if("rescan".equals(action)) {
             getEngine().findJobConfigs(); 
         } else if ("add".equals(action)) {
-            getEngine().considerAsJobDirectory(new File(form.getFirstValue("addpath")));
+            String path = form.getFirstValue("addpath");
+            if(path==null) {
+                path = "";
+            }
+            boolean added = false;
+            if(StringUtils.isNotBlank(path)) {
+                added = getEngine().considerAsJobDirectory(new File(path));
+            }
+            if(!added) {
+                Flash.addFlash(getResponse(), "not a valid preexisting job directory: \""+path+"\"", Flash.Kind.NACK);
+            }
         }
         // default: redirect to GET self
         getResponse().redirectSeeOther(getRequest().getOriginalRef());
@@ -98,6 +109,8 @@ public class EngineResource extends Resource {
         pw.println("</head><body>");
         pw.println("<h1>"+engineTitle+"</h1>"); 
         
+        Flash.renderFlashesHTML(pw, getRequest());
+
         pw.println("<b>Memory: </b>");
         pw.println(engine.heapReport());
         pw.println("<br/><br/>");