sftp should fail on error

[chrisse27]

Currently sftp only outputs a warning in errors such as "Permission denied":

02-Jul-2015 16:19:16    Running "sftp:uploadCoreConfiguration" (sftp) task
02-Jul-2015 16:19:17    >> Error: Permission denied
...
02-Jul-2015 16:19:18    Done, without errors.

This is due

        }, function (err) {
          if (err) {
            grunt.fail.warn(err);
          }
          c.end();
        });
      });

in line 361.

sftp should fail on errors of this kind.