Fix shutdown exc

[ixje]

What current issue(s) does this address, or what feature is it adding?
if the nodemanager port is already in use (e.g. due to running a private net on the same port) then we will abort and shutdown. However, shutdown() can be called before all tasks have been started. The asyncio.gather will receive None type instead of a task which will throw an exception.

How did you solve this problem?
sanity check that a task exists before adding it to list of tasks we wait for during shutdown.

How did you make sure your solution works?
manually testing

Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• [] Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage increased (+0.002%) to 85.123% when pulling 979c2e4 on ixje:fix-shutdown-exc into d61452a on CityOfZion:development.