ttools: downgrade taplint DQU2 message from Error to Warning

Tom McGlynn points out that use of an unknown (not ERROR/OVERFLOW)
value in a post-table QUERY_STATUS INFO element is not strictly
prohibited by TAP (or DALI).  CDA are noting value="OK" here.
So I've downgraded it to a Warning.

ttools/src/docs/sun256.xml

@@ -8697,6 +8697,8 @@ eds. C. Gabriel et al., ASP Conf. Ser. 351, p. 666 (2006)
             <code><ref id="mode-gui">gui</ref></code>,
             which displays the table data in a scrollable
             window on the screen.</li>
+        <li>taplint: downgrade unknown post-table QUERY_STATUS value message
+            from Error to Warning.</li>
         </ul>
         </p></dd>
 

ttools/src/main/uk/ac/starlink/ttools/taplint/FixedCode.java

@@ -54,7 +54,6 @@ public enum FixedCode implements ReportCode {
     E_DFIO( "Duff query failed" ),
     E_DFSF( "Duff query result parse failure" ),
     E_DNST( "Missing QUERY_STATUS from duff query" ),
-    E_DQU2( "Unknown value for post-table QUERY_STATUS" ),
     E_DQUS( "Unknown value for QUERY_STATUS" ),
     E_ELDF( "Wrong top-level element" ),
     E_EST1( "Multiple QUERY_STATUS INFOs" ),
@@ -147,6 +146,7 @@ public enum FixedCode implements ReportCode {
     W_CULF( "Custom language feature type" ),
     W_CUNI( "Unit mismatch" ),
     W_CUTP( "UType mismatch" ),
+    W_DQU2( "Unknown value for post-table QUERY_STATUS" ),
     W_DSUC( "Non-error return from duff query" ),
     W_FKUN( "Keys content" ),
     W_FLUN( "Key columns content" ),

ttools/src/main/uk/ac/starlink/ttools/taplint/VotLintTapRunner.java

@@ -398,7 +398,7 @@ else if ( "OVERFLOW".equals( postStatus ) ) {
                         .append( postStatus )
                         .append( " is not ERROR/OVERFLOW" )
                         .toString();
-                    reporter.report( FixedCode.E_DQU2, msg );
+                    reporter.report( FixedCode.W_DQU2, msg );
                 }
             }
         }