Better task log errors.

indexing-service/src/main/java/io/druid/indexing/overlord/http/OverlordResource.java

@@ -398,7 +398,12 @@ public class OverlordResource
       if (stream.isPresent()) {
         return Response.ok(stream.get().getInput()).build();
       } else {
-        return Response.status(Response.Status.NOT_FOUND).build();
+        return Response.status(Response.Status.NOT_FOUND)
+                       .entity(
+                           "No log was found for this task. "
+                           + "The task may not exist, or it may not have begun running yet."
+                       )
+                       .build();
       }
     }
     catch (Exception e) {

s3-extensions/src/main/java/io/druid/storage/s3/S3TaskLogs.java

@@ -97,8 +97,9 @@ public class S3TaskLogs implements TaskLogs
       );
     }
     catch (ServiceException e) {
-      if (e.getErrorCode() != null && (e.getErrorCode().equals("NoSuchKey") || e.getErrorCode()
-                                                                                .equals("NoSuchBucket"))) {
+      if (404 == e.getResponseCode()
+          || "NoSuchKey".equals(e.getErrorCode())
+          || "NoSuchBucket".equals(e.getErrorCode())) {
         return Optional.absent();
       } else {
         throw new IOException(String.format("Failed to stream logs from: %s", taskKey), e);