Only monitor events for a valid device ID (#30)

[damonbarry]

Reference/Link to the issue solved with this PR (if any)

Solves #30

Description of the problem

iothub-explorer will happily monitor events for a device that doesn't exist:

$ iothub-explorer monitor-events i-do-not-exist -l "{connection-string}"

Monitoring events from device i-do-not-exist...

Description of the solution

Now the monitor-events command checks that the device exists before setting up the event monitor. If it does not exist, the command fails:

$ iothub-explorer monitor-events i-do-not-exist -l "{connection-string}"

Error: DeviceNotFoundError: i-do-not-exist

[pierreca]