Deployment of plugin fails

## Gentics Mesh Version, operating system, or hardware.
- v1.1.0

## Problem
When the deployment of a plugin (e.g. on mesh startup) fails due to an error during initialization (e.g. plugin expects a project to exist, which does not exist yet), the plugin cannot be deployed over the /admin/plugins endpoint with a "generic_error".

Mesh log shows:

08:44:13.654 [] ERROR [vert.x-worker-thread-5] - Plugin deployment of {mesh-formgenerator-plugin-1.0.1.jar} failed.
cr_1 | org.pf4j.PluginAlreadyLoadedException: Plugin '/plugins/mesh-formgenerator-plugin-1.0.1.jar' already loaded with id 'fg'

It seems that the plugin is still somehow registered, even if it was not deployed on startup

## Reproducer

1. Setup new mesh instance with the formgenerator plugin placed in the plugins folder (with existing config.yml that references the project "fg-project")
2. Start mesh instance -> plugin deployment fails (fg-project not found)
3. Create fg-project
4. Deploy plugin over /admin/plugins endpoint

## Expected behaviour and actual behaviour

Expected: plugin deployment should work
Actual: plugin deployment fails (until mesh is restarted)