Handle SQL API errors by logging them and requesting NO cache

SQL api is used to determine the list of source tables affected
by a query. Before this commit, the X-Cache-Channel header set
on sql api error was an arbitrary 'table' string, now the header
is omitted, the error logged and Cache-Control and Pragma headers
are sent as an attempt to request no caching.

The code includes test for this mechanism.

lib/cartodb/cache_validator.js

@@ -36,7 +36,7 @@ function generateCacheChannel(req, callback){
 
         // use cache if present
         if (!_.isNull(channelCache[sql_md5]) && !_.isUndefined(channelCache[sql_md5])) {
-            callback(channelCache[sql_md5]);
+            callback(null, channelCache[sql_md5]);
         } else{
             // strip out windshaft/mapnik inserted sql if present
             var sql = req.params.sql.match(/^\((.*)\)\sas\scdbq$/);
@@ -54,21 +54,28 @@ function generateCacheChannel(req, callback){
             }
 
             // call sql api
-            request.get({url:sqlapi, qs:qs, json:true}, function(err, response, body){
-                if (!err && response.statusCode == 200) {
-                    tableNames = body.rows[0].cdb_querytables.split(/^\{(.*)\}$/)[1];
-                } else {
-                    //oops, no SQL API. Just cache using fallback 'table' key
-                    tableNames = 'table';
+            request.get({url:sqlapi, qs:qs, json:true}, function(err, res, body){
+                var epref = 'could not detect source tables using SQL api at ' + sqlapi;
+                if (err){
+                  var msg = err.message ? err.message : err;
+                  callback(new Error(epref + ': ' + msg));
+                  return;
                 }
+                if (res.statusCode != 200) {
+                    var msg = res.body.error ? res.body.error : res.body;
+                    callback(new Error(epref + ': ' + msg));
+                    return;
+                } 
+                var qtables = body.rows[0].cdb_querytables;
+                tableNames = qtables.split(/^\{(.*)\}$/)[1];
                 cacheChannel = buildCacheChannel(dbName,tableNames);
                 channelCache[sql_md5] = cacheChannel; // store for caching
-                callback(cacheChannel);
+                callback(null, cacheChannel);
             });
         }
     } else {
         cacheChannel = buildCacheChannel(dbName,tableNames);
-        callback(cacheChannel);
+        callback(null, cacheChannel);
     }
 }