aioble: Fix missing GattError import in server.py.

2015-01-01 07:45:46.790 INFO Received OLCP_OP_FIRST
2015-01-01 07:45:46.979 ERROR Error in ble_write_listener
Traceback (most recent call last):
  File "aioble/server.py", line 223, in indicate
NameError: name 'GattError' isn't defined

Author: peitschie

micropython/bluetooth/aioble/aioble/server.py

@@ -13,6 +13,7 @@
     log_error,
     log_warn,
     register_irq_handler,
+    GattError,
 )
 from .device import DeviceConnection, DeviceTimeout