make error message more informative

When one passes a float, let's say -m=2.5G the only message that is returned is: "sambamba-sort: couldn't parse ", which is not very informative, as this someone may blame the input BAM file and spend some time trying to figure out what is wrong with it. The updated  error message points directly into the point saving users time.

sambamba/sort.d

@@ -604,7 +604,7 @@ size_t parseMemory(string str) {
         case "GB":
             return sz * 1_000_000_000;
         default:
-            throw new Exception("couldn't parse ", initial_str);
+            throw new Exception("couldn't parse --memory-limit option ", initial_str);
     }
 }