Description of problem:
If the target playbook can't be found, it is better to provide a meaningful messages rather than throw exception

Version-Release number of selected component (if applicable):
atomic-openshift-installer

How reproducible:
always

Steps to Reproduce:
1. specify  variant_version: '3.2' or variant_version to unsupported values
2. try to upgrade to the latest version

Actual results:
atomic-openshift-installer -c config/hostmini.yaml upgrade

        This tool will help you upgrade your existing OpenShift installation.

Version 3.2 found. Do you want to update to the latest version of 3.2 or migrate to the next major release?
(1) Update to latest 3.2 (2) Migrate to next release: 1
Traceback (most recent call last):
  File "/bin/atomic-openshift-installer", line 9, in <module>
    load_entry_point('ooinstall==3.0.0', 'console_scripts', 'oo-install')()
  File "/usr/lib/python2.7/site-packages/click/core.py", line 664, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 644, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 991, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python2.7/site-packages/click/core.py", line 837, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 464, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/ooinstall/cli_installer.py", line 839, in upgrade
    playbook = mapping['minor_playbook']
TypeError: 'NoneType' object has no attribute '__getitem__'

Expected Result:
it is better to display human language like 'upgrade from openshift-enterprise 3.2 to latest is unsupported' rather than throw exception.

Additional info: