(PDK-415) Convert user-input related problems from FATAL to ERROR

[rodjek]

Adds a new exception type that should be raised when dealing with user input related problems (like trying to create a class that already exists).

asmodean :0: pdk/foo (git:pdk-415 → origin {2} ?:2!)$ ../bin/pdk new class foo
pdk (ERROR): Unable to generate class; '/home/tsharpe/code/puppet/pdk/foo/manifests/init.pp' already exists.

PDK::CLI::FatalError should still be used when catching and re-raising an exception, as those are almost always fatal errors and should be displayed as such.

[DavidS]

Code looks good, rekicked COVERAGE cell on travis to resuscitate coveralls.

[DavidS]

Merged without coverage report

[coveralls]

Coverage decreased (-0.4%) to 93.685% when pulling a3cced5 on rodjek:pdk-415 into e7ec32a on puppetlabs:master.