Better error message when using a parameter that is not defined

[jsyrjala]

Currently the error message looks like this (ndt 1.0a31)

Traceback (most recent call last):
  File "/usr/local/bin/cf-update-stack", line 11, in <module>
    sys.exit(update_stack())
  File "/usr/local/lib/python3.6/site-packages/n_utils/cli.py", line 489, in update_stack
    args.dry_run)
  File "/usr/local/lib/python3.6/site-packages/n_utils/cf_deploy.py", line 271, in deploy
    for key in list(template_parameters.keys()):
AttributeError: 'list' object has no attribute 'keys'

[jsyrjala]

The same error message seems to also happen when the stack doesn't have any parameters (and stack doesn't use any parameters).

[jsyrjala]

Happens also in 1.0a47