Update error for requirement of S3BackEnd

git-fat

@@ -146,7 +146,7 @@ except ImportError:
     class S3Backend(object):
 
         def __init__(self,bucket,key,secret,objdir):
-            raise RuntimeError("S3Backend requires boto.")
+            raise RuntimeError("S3Backend requires boto and filechunkio.")
 
 BLOCK_SIZE = 4096
 LARGE_FILE_SIZE = 4294967296    # consider > 4GBs is large file size(Current limitation in windows)