Fix error messages for host assignment

This does 2 things for the host assignment error message reporting:
1) changes flash :warning to flash :error:
     This is a real error condition, so users should be properly
     alerted
2) Display the actual error messages to the user rather than burying
   them in logs:
     There are a variety of different conditions that can lead to
     host assignment failing, from UI-fixable stuff around setting
     operating system, etc. params in foreman to lower-level problems
     with DNS, DHCP, etc.

Updated commit resolves the formatting issues