use format-error for rendering errors

New output format is based on the [format-error](https://github.com/EndangeredMassa/format-error) module.

```
Error:  Process "testium proxy" crashed. See log at: /home/sean/demo/testium/test/integration_log/proxy.log.
  at procError (/home/sean/demo/testium/lib/selenium/process/port.js:47:13)
  at /home/sean/demo/testium/lib/selenium/process/port.js:64:19
  at Socket.<anonymous> (/home/sean/demo/testium/node_modules/portscanner/lib/portscanner.js:81:5)
  at Socket.EventEmitter.emit (events.js:95:17)
  at TCP.close (net.js:465:12)

stderr: 
/home/sean/demo/testium/lib/proxy/index.js:49
  throw new Error('test error');
        ^
Error: test error
    at Object.<anonymous> (/home/sean/demo/testium/lib/proxy/index.js:49:9)
    at Object.<anonymous> (/home/sean/demo/testium/lib/proxy/index.js:51:3)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:902:3
```