Error out a bit more nicely if the RewriteMap prg: is not found. We

Error out a bit more nicely if the RewriteMap prg: is not found.  We
can't just apr_stat in the first init round because we haven't run
apr_tokenize_to_argv() yet, and it would be a relatively ugly hack to run
it twice just for that.  Well, I suppose we could store the argv in the
rewritemap structure, but ... nah.  With this, we shutdown (cleanly, as
opposed to the old exit(1) method) if we go to execute a rewritemap
and discover it doesn't exist, and log a nice descriptive message at the
end of the error_log.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95337 13f79535-47bb-0310-9956-ffa450edef68