dspace-api: improve logging in ImageMagickThumbnailFilter

Instead of logging the name of the source bitstream, we should be
logging the name of the actual thumbnail bitstream that is being
considered for replacement. For example, instead of this:

  IM Thumbnail manual.pdf matches pattern and is replaceable.

... the message should read:

  IM Thumbnail manual.pdf.jpg matches pattern and is replaceable.

This message is already confusing enough, but this will help.

dspace-api/src/main/java/org/dspace/app/mediafilter/ImageMagickThumbnailFilter.java

@@ -209,14 +209,14 @@ public boolean preProcessBitstream(Context c, Item item, Bitstream source, boole
                     if (replaceRegex.matcher(description).matches()) {
                         if (verbose) {
                             System.out.format("%s %s matches pattern and is replaceable.%n",
-                                    description, nsrc);
+                                    description, n);
                         }
                         continue;
                     }
                     if (description.equals(getDescription())) {
                         if (verbose) {
                             System.out.format("%s %s is replaceable.%n",
-                                    getDescription(), nsrc);
+                                    getDescription(), n);
                         }
                         continue;
                     }