building error map fails when backend is offline

[jyu00]

Information

• Qiskit IBMQ Provider version:

• API URL or version:

• Python version:

• Operating system:

What is the current behavior?

2 tests currently fail because ibmqx2 and ibmq_16_melbourne are offline:

ERROR: test_error_map_tab (test.ibmq.test_jupyter.TestBackendInfo)
  File "/home/travis/build/IBM-Q-Software/sw-qiskit-ibmq-provider/qiskit-ibmq-provider/qiskit/providers/ibmq/visualization/interactive/error_map.py", line 171, in iplot_error_map
    vmin=min(cx_errors[cx_idx]), vmax=max(cx_errors[cx_idx]))
ValueError: min() arg is an empty sequence

ERROR: test_backend_widget (test.ibmq.test_jupyter.TestIQXDashboard)
  File "/home/travis/build/IBM-Q-Software/sw-qiskit-ibmq-provider/qiskit-ibmq-provider/qiskit/providers/ibmq/jupyter/dashboard/backend_widget.py", line 140, in make_backend_widget
    avg_cx_err = round(sum_cx_err/(num_cx)*100, 2)
ZeroDivisionError: division by zero

Steps to reproduce the problem

Run the tests.

What is the expected behavior?

The jupyter widgets should not error even when the backend is reporting 100% cx error.

Suggested solutions