Add context to TimeoutErrors (#201)

`ChargePoint.call()` raises a `asyncio.TimeoutError` if a call isn't
answered withing a given time.

However, the `asyncio.TimeoutError` is missing context. The lack of
context makes interpreting this exception hard than it should be.

This commit add that context.

Fixes: #200

ocpp/charge_point.py

@@ -260,9 +260,15 @@ async def call(self, payload, suppress=True):
         # a time.
         async with self._call_lock:
             await self._send(call.to_json())
-            response = \
-                await self._get_specific_response(call.unique_id,
-                                                  self._response_timeout)
+            try:
+                response = \
+                    await self._get_specific_response(call.unique_id,
+                                                      self._response_timeout)
+            except asyncio.TimeoutError:
+                raise asyncio.TimeoutError(
+                    f"Waited {self._response_timeout}s for response on "
+                    f"{call.to_json()}."
+                )
 
         if response.message_type_id == MessageType.CallError:
             LOGGER.warning("Received a CALLError: %s'", response)