shill_proxy: use ShillProxyError

While attempting to raise an error in another shill_proxy CL, noticed
that the error.TestFail class was not available in this context.  Fix by
using the Exception class defined in shill_proxy.

BUG=None
TEST=None

Change-Id: I0affd21eda5497570ab810d91a1995c7e360ddcb
Signed-off-by: Jason Abele <jabele@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/65016
Reviewed-by: Christopher Wiley <wiley@chromium.org>
diff --git a/test-scripts/shill_proxy.py b/test-scripts/shill_proxy.py
index 224d0a0..807a586 100644
--- a/test-scripts/shill_proxy.py
+++ b/test-scripts/shill_proxy.py
@@ -163,7 +163,7 @@
                 connection = cls()
             except dbus.exceptions.DBusException as e:
                 if e.get_dbus_name() != ShillProxy.DBUS_SERVICE_UNKNOWN:
-                    raise error.TestFail('Error connecting to shill')
+                    raise ShillProxyError('Error connecting to shill')
                 else:
                     # Wait a moment before retrying
                     time.sleep(ShillProxy.POLLING_INTERVAL_SECONDS)