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.

One remaining issue -- this isn't formatted in the best possible way.
If there are multiple error messages, the list gets a bit ugly,
but it's better than not providing any messages at all. If there's
an obvious way to better format this for the flash[:error] array, then
let me know -- otherwise perhaps this is good enough for now, as it's
a definite improvement over what we had previously.