Handle error from neutron extension check in Overview page

Previously some Neutron API calls in get_neutron_limits are outside
try/except and if an error occurs a traceback will be displayed.

Change-Id: I191aecbf3135ddac3b20dbc7e0f22b63aa3837f9
Closes-Bug: #1252878
Merge "Handle error from neutron extension check in Overview page"