Pass a partially received response in the ValueError exception.

Allow better debugging when an incomplete response was received or
timed out.  The already received part (empty on timeout) is passed as
the first and only argument in the ValueError exception, accessible as
e.args[0] in a handler.

umodbus/utils.py

@@ -138,6 +138,6 @@ def recv_exactly(recv_fn, size):
     response = b''.join(chunks)
 
     if len(response) != size:
-        raise ValueError
+        raise ValueError(response)
 
     return response