Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1173617
Description of problem:
If one tries changing Puppet environment of a host to an environment which doesn't contain a puppet module which is assigned to the host, the operation will silently fail without any explanation in the UI.
In the production.log, the following line can be seen:

Failed to save: Puppetclasses puppet_dev does not belong to the KT_acme_it_Prototype_ACME_IT_Sample_Service_13 environment

Version-Release number of selected component (if applicable):
satellite 6.0.6

Steps to Reproduce:
1. Create a host with some puppet class assigned and a smart-parameter set
2. Edit the host and change to a Puppet Environment where said puppet class isn't present
3. Press Submit
4. Note how you are taken back to the "edit host" screen without any explanation