Plugin install fails silently

[w33ble]

When installing plugins via the config, installation failures are silent and ESVM starts the cluster without indicating a problem:

Above, my config is trying to install a plugin named thisisnotavalidplugin, which is, in fact, not a real plugin, as confirmed with the plugin install tool in ES.

The exit code from the CLI tool is non-zero, so libesvm should be able to tell that the install failed.