rax:message extension

When checking plain parameters,  the rax:message extension should supply an appropriate human readable message.

Example:

````xml
<param name="usage"
              style="plain"
              required="true"
              path="if (/atom:entry/atom:content/event:event[@type='USAGE']) then (/atom:entry/atom:content/event:event[@type='USAGE']/w_ns3:product,/atom:entry/atom:content/event:event[@type='USAGE']/w_ns4:product,/atom:entry/atom:content/event:event[@type='USAGE']/w_ns5:product,/atom:entry/atom:content/event:event[@type='USAGE']/w_ns6:product) else true()"
              rax:message="Only usage messages with product attributes in the following namespaces are allowed in this feed: http://docs.rackspace.com/usage/cloudbackup/bandwidthIn, http://docs.rackspace.com/usage/cloudbackup/bandwidthOut, http://docs.rackspace.com/usage/cloudbackup/license, http://docs.rackspace.com/usage/cloudbackup/storage"/>
````