Improve error message in init_rate_histogram()

This CL improves the error message for the allocation failure in
init_rate_histogram() as suggested in
https://aomedia-review.googlesource.com/c/aom/+/184121

Bug: aomedia:3276
Change-Id: I728824553f4c22bb2d89383be8deb20249467351
diff --git a/stats/rate_hist.c b/stats/rate_hist.c
index d851d15..d79ebc5 100644
--- a/stats/rate_hist.c
+++ b/stats/rate_hist.c
@@ -69,7 +69,9 @@
 
 fail:
   fprintf(stderr,
-          "Warning: unable to allocate histogram buffers. Continuing...\n");
+          "Warning: Unable to allocate buffers required for "
+          "show_rate_histogram().\n"
+          "Continuing without rate histogram feature...\n");
   destroy_rate_histogram(hist);
   return NULL;
 }