(maint) Remove redundant methods and refactor.

Before this commit the Puppet::Environments::Cached#get! was not needed
as it was defined in a parent class. Puppet::Environments::Combined#get!
now calls get! in environment objects instead of assuming how get!
should work.

This commit also refactors areas in the code where
Puppet::Environments::EnvironmentNotFound was raised after calling get
in favor of the new get! method.