[lldb] Improve error reporting from TestAppleSimulatorOSType.py

When we can't find a simulator, report the platform and architecture in
the error message.

lldb/test/API/tools/lldb-server/TestAppleSimulatorOSType.py

@@ -49,7 +49,7 @@ class TestAppleSimulatorOSType(gdbremote_testcase.GdbRemoteTestCaseBase):
                 break
 
         # Launch the process using simctl
-        self.assertIsNotNone(deviceUDID)
+        self.assertIsNotNone(deviceUDID, 'Could not find a simulator for {} ({})'.format(platform_name, arch))
 
         exe_name = 'test_simulator_platform_{}'.format(platform_name)
         sdkroot = lldbutil.get_xcode_sdk_root(sdk)