plugin -remove deletes bin directory
I tried to install a plugin directly from disk, but used the wrong arguments. Totally my fault for not RTFM properly.

However, by following the instructions printed out by `bin/plugin`, I ended up deleting my $ES/bin directory.

```
$ pwd
/tmp/elasticsearch-0.90.5
$ ls
LICENSE.txt    NOTICE.txt     README.textile bin            config         lib
$ bin/plugin --install file:///tmp/foo.zip
-> Installing file:///tmp/foo.zip...
Failed to install file:///tmp/foo.zip, reason: plugin directory /tmp/elasticsearch-0.90.5/plugins already exists. To update the plugin, uninstall it first using -remove file:///tmp/foo.zip command
$ bin/plugin -remove file:///tmp/foo.zip
-> Removing file:///tmp/foo.zip
Removed file:///tmp/foo.zip
$ ls
LICENSE.txt    NOTICE.txt     README.textile config         lib
```

I reproduced the problem in 0.90.5 and the latest master.
Closes #3847.