(PDK-809) Exit early if the module is not PDK compatible

[rodjek]

The current behaviour if you run pdk validate or pdk test unit on a module that hasn't been converted to be PDK compatible is to display a warning and attempt to continue anyway.

semirhage :0: pdk/foo (git:master → origin ↑10 ?:1!)$ ../bin/pdk validate
pdk (WARN): This module is not PDK compatible. Run `pdk convert` to make it compatible with your version of PDK.
pdk (INFO): Running all available validators...
pdk (INFO): Using Ruby 2.4.4
pdk (INFO): Using Puppet 5.5.1
[✔] Resolving default Gemfile dependencies.
[✔] Checking metadata syntax (metadata.json tasks/*.json).
[✔] Checking module metadata style (metadata.json).
[✔] Checking Ruby code style (**/**.rb).
info: task-metadata-lint: ./: Target does not contain any files to validate (tasks/*.json).
info: puppet-syntax: ./: Target does not contain any files to validate (**/**.pp).
info: puppet-lint: ./: Target does not contain any files to validate (**/*.pp).

With this change, the PDK will instead display the message and exit early.

semirhage :0: pdk/foo (git:pdk-809 U:2 ?:1!)$ ../bin/pdk validate
pdk (ERROR): This module is not PDK compatible. Run `pdk convert` to make it compatible with your version of PDK.

[coveralls]

Coverage decreased (-0.003%) to 92.844% when pulling 6fecd34 on rodjek:pdk-809 into 69f4332 on puppetlabs:master.