"nova boot" should not log an error if subsidiary commands fail

Fix:
Intially the cli was raising "CommadError" in case the requested
flavor or image were not present.
This error category was not approrpiate as it signifies an error
in the command syntax. When the requested resource (flavour/image)
does not exist, a ResourceNotFound error should be raised. So,
added a new error category "ResourceNotFound" to cater for this
scenario and updated the code to raise this new error.
"nova show <instance_name>" command has also been updated to raise
"ResourceNotFound" error when the requested vm for which details
have to be displayed does not exist.

Closes-Bug: #1258488
Change-Id: If64a087944586ef5792efe3baa62e455b9bbaa07
Merge ""nova boot" should not log an error if subsidiary commands fail"