add getRequiredParameter() to SolrParams

I find myself including this with every patch, so i'll just separate it out. This simply adds a utilty function to SolrParams that throws a 400 if the parameter is missing:

/** returns the value of the param, or throws a 400 exception if missing */
public String getRequiredParameter(String param) throws SolrException {
String val = get(param);
if( val == null )

{ throw new SolrException( 400, "Missing parameter: "+param ); }

return val;
}