Fix for issue jMonkeyEngine#2302 - NativeLibraryLoader fails due to no write permissions

Extract native library to a folder directly beneath the user temp folder. Do not use an intermediate /jme subfolder.
Check the result of the call to mkdir. Use UserCache if creation failed.
Check if the extraction folder is actually writable. If not use the UserCache.
Updated log message: since a previous change we no longer extract to the working directory.

Author: andreas

jme3-desktop/src/main/java/com/jme3/system/NativeLibraryLoader.java

@@ -198,14 +198,15 @@ public static File getExtractionFolder() {
                 setExtractionFolderToUserCache();
             } else {
                 try {
-                    File jmeTempDir = new File(userTempDir, "jme3");
-                    if (!jmeTempDir.exists()) {
-                        jmeTempDir.mkdir();
-                    }
-                    extractionFolder = new File(jmeTempDir, "natives_" + Integer.toHexString(computeNativesHash()));
+                    extractionFolder = new File(userTempDir, "jme3_natives_" + Integer.toHexString(computeNativesHash()));
 
                     if (!extractionFolder.exists()) {
-                        extractionFolder.mkdir();
+                        if(!extractionFolder.mkdir()) {
+                            throw new IOException("Failed to create folder "+extractionFolder);
+                        }
+                    }
+                    if(!extractionFolder.canWrite()) {
+                        setExtractionFolderToUserCache();
                     }
                 } catch (Exception e) {
                     setExtractionFolderToUserCache();
@@ -268,7 +269,7 @@ private static void setExtractionFolderToUserCache() {
             extractionFolder.mkdir();
         }
 
-        logger.log(Level.WARNING, "Working directory is not writable. "
+        logger.log(Level.WARNING, "Temp directory is not writable. "
                                 + "Natives will be extracted to:\n{0}", 
                                 extractionFolder);
     }