When the plugin manager does not find in `plugin-descriptor.properties` the exact same elasticsearch version it was built on
as the current elasticsearch version, it fails with a message like:

```
ERROR: Elasticsearch version [2.0.0-beta1] is too old for plugin [elasticsearch-mapper-attachments]
```

Actually, the message should be:

```
Plugin [elasticsearch-mapper-attachments] is incompatible with Elasticsearch [2.0.0.beta2]. Was designed for version [2.0.0.beta1].
```

The opposite is true. If you try to install a version of a plugin which was built with a newer version of elasticsearch, it will fail the same way:

```
Plugin [elasticsearch-mapper-attachments] is incompatible with Elasticsearch [2.0.0.beta1]. Was designed for version [2.0.0.beta2].
```

(cherry picked from commit 28f82fb)
(cherry picked from commit 9886abd)
(cherry picked from commit 6138450)