Check the return value of `delete` calls

Signed-off-by: Andrew DeMaria <lostonamountain@gmail.com>

airsonic-main/src/main/java/org/airsonic/player/ajax/CoverArtService.java

@@ -160,7 +160,9 @@ public class CoverArtService {
     private void backup(File newCoverFile, File backup) {
         if (newCoverFile.exists()) {
             if (backup.exists()) {
-                backup.delete();
+                if (!backup.delete()) {
+                    LOG.warn("Failed to delete " + backup);
+                }
             }
             if (newCoverFile.renameTo(backup)) {
                 LOG.info("Backed up old image file to " + backup);

airsonic-main/src/main/java/org/airsonic/player/filter/BootstrapVerificationFilter.java

@@ -90,8 +90,7 @@ public class BootstrapVerificationFilter implements Filter {
     private boolean directoryWritable(File dir) {
         try {
             File tempFile = File.createTempFile("test", null, dir);
-            tempFile.delete();
-            return true;
+            return tempFile.delete();
         } catch (IOException x) {
             return false;
         }

airsonic-main/src/main/java/org/airsonic/player/service/PodcastService.java

@@ -579,7 +579,9 @@ public class PodcastService {
                 if (isEpisodeDeleted(episode)) {
                     LOG.info("Podcast " + episode.getUrl() + " was deleted. Aborting download.");
                     FileUtil.closeQuietly(out);
-                    file.delete();
+                    if (!file.delete()) {
+                        LOG.warn("Unable to delete " + file);
+                    }
                 } else {
                     addMediaFileIdToEpisodes(Arrays.asList(episode));
                     episode.setBytesDownloaded(bytesDownloaded);