Check Plug-In Validity When Loading

**Is your feature request related to a problem? Please describe.**
If there's a problem with the code of a plugin, we should not crash our whole application because of this.

**Describe the solution you'd like**
Define a boundary when loading plugins into the system that validates the plugin state on the assumption the plugin author messed up somehow. We should display this (and the error) on the plugins view, so that the user has some idea of what has gone wrong.