Don't reuse connections if the reader has an exception.

Author: TimothyFitz

aiomysql/pool.py

@@ -148,7 +148,7 @@ async def _fill_free_pool(self, override_min):
         n = 0
         while n < free_size:
             conn = self._free[-1]
-            if conn._reader.at_eof():
+            if conn._reader.at_eof() or conn._reader.exception():
                 self._free.pop()
                 conn.close()