Description of problem:
This is an extension of bug 867772 to behave similarly to unix command line programs that print basic usage to stderr if they are called incorrectly.

This is mainly useful for people building stuff on top of rest API or people who have to use REST API for debugging because in such cases, typos or forgetting exact names are inevitable and this reduces times spent by digging in documentation greatly.

Version-Release number of selected component (if applicable):
3.1.0-18 / si19.1

How reproducible:
always

Steps to Reproduce:
1. run curl ... -X POST "garbage" https://rhevm.example.org/api/vms/<vm_uuid>/ticket
2.
3.
  
Actual results:
generic HTTP error page is served

Expected results:
include correct usage information. Given that the reply is itself XML, it could include subtree like this (I may request too verbose answer but I can imagine that printing defaults could save considerable effort on debugging of some issues):

<action>
    ...
    <correct-usage>
        <ticket>
            <!-- mandatory -->
            <expiry>
                <!-- integer, optional: number of seconds when password
                     is valid. Default: 7200, Default in Portal: 120 -->
            </expiry>
        </ticket>
    </correct-usage>
</action>

Additional info: