Properly check if connection can be established for importing interactive visualizations

[nonhermitian]

Fix #895

Summary

As pointed out by @chriseclectic , loading qiskit with no internet connection results in a ConnectionError because the interactive visualizations max out the number of attempts to look for the needed host url. Here we replace the previous check with on that gracefully exits if no connection can be established.

Details and comments

[delapuente]

Please, remember to file an issue and link the PR with it by adding "Fix <issue_number>".

[diego-plan9]

Can you remember to remove requests as a dependency, as the only place where it was used is now replaced by a standard library call (which I think is a good move)?

[diego-plan9]

Also, perhaps we could take the chance to reevaluate the need for the

from qiskit.tools.visualization import (circuit_drawer, plot_histogram)

line from the main qiskit/__init__.py, in the spirit of @ajavadia's comment at #890 (comment)?

Our top-level qiskit namespace and import qiskit is already more complicated and full of implications than ideal, and importing a member of qiskit.tools there entangles (no pun intended) it even more, which has been the root cause of two user-facing bugs (#895 and #889) already since it was introduced. Any thoughts?

[nonhermitian]

Actually the bug fixed here was caught becuase of the top level visualization imports. They were not the cause. Anytime someone loaded the visualization module, the same issue would pop up.

[nonhermitian]

The function added here could also be useful for having a more graceful error message if the register function cannot connect to the host urls.

[mtreinish]

This LGTM, I was going to fix it by just catching requests.exceptions.RequestException but switching to just checking if we can open a socket or not works fine too.