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.

config/environments/test.js.example

@@ -48,8 +48,10 @@ var config = {
     }
     ,sqlapi: {
         protocol: 'http',
-        host: 'localhost.lan',
-        port: 8080,
+        host: '',
+        // This port will be used by "make check" for testing purposes
+        // It must be available
+        port: 1080,
         version: 'v1'
     }
     ,varnish: {