Change error messages when getting overviews metadata fails

Remove the detail that the error occurred trying to get overviews
metadata from the error message. This should be less confusing
to the user.

lib/cartodb/api/overviews_metadata_api.js

@@ -26,7 +26,7 @@ OverviewsMetadataApi.prototype.getOverviewsMetadata = function (username, sql, c
     this.pgQueryRunner.run(username, query, function handleOverviewsRows(err, rows) {
         if (err){
             var msg = err.message ? err.message : err;
-            callback(new Error('could not get overviews metadata: ' + msg));
+            callback(new Error(msg));
             return;
         }
         var metadata = {};

test/acceptance/multilayer_server.js

@@ -334,7 +334,7 @@ describe('tests from old api translated to multilayer', function() {
 
                 var parsed = JSON.parse(res.body);
                 assert.deepEqual(parsed, {
-                    errors: ["could not get overviews metadata: fake error message"]
+                    errors: ["fake error message"]
                 });
 
                 done();