Improve error message "Not all nodes are accounted for in the --node-disks dictionary"

[jarcec]

As it stands today one has to open the source code in order to understand what does this error mean:

ubuntu@ip-172-31-12-226:~$ clusterdock start topology_mapr --namespace streamsets --node-disks='{node-1:[/dev/xvdb],node-2:[/dev/xvdc],node-3:[/dev/xvdd]}' --predictable --mapr-version 5.2.2 --mep-version 3.0.1
Traceback (most recent call last):
  File "/usr/local/bin/clusterdock", line 9, in <module>
    load_entry_point('clusterdock==1.2.0', 'console_scripts', 'clusterdock')()
  File "/usr/local/lib/python3.5/dist-packages/clusterdock/cli.py", line 175, in main
    action.main(args)
  File "/home/ubuntu/topology_mapr/start.py", line 41, in main
    raise Exception('Not all nodes are accounted for in the --node-disks dictionary')
Exception: Not all nodes are accounted for in the --node-disks dictionary

Would it make sense to perhaps improve the error message and say what nodes are missing or extra?