Plugins: NPE when plugins dir is inaccessible

Steps to reproduce:

1. Download fresh es.
2. `sudo mkdir plugins && sudo chmod 0700 plugins`
3. Start elasticsearch

```
elasticsearch-1.4.1 λ ./bin/elasticsearch
[2014-12-09 12:18:59,025][INFO ][node                     ] [Piotr Rasputin] version[1.4.1], pid[16338], build[89d3241/2014-11-26T15:49:29Z]
[2014-12-09 12:18:59,025][INFO ][node                     ] [Piotr Rasputin] initializing ...
{1.4.1}: Initialization Failed ...
- NullPointerException[null]
```

Closes #8837.
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