Attempting to load a plugin that doesn't exist on disk crashes OmniSharp.

[NTaylorMullen]

This is an edge case because trying to load a plugin that doesn't exist means that the user did:

1. Manually deleted the locally downloaded omnisharp and/or the plugin
2. Initial acquisition was corrupt

I tried fixing this on my own here but for some assembly loading reason it results in the plugin no longer being valid and not being found by the composition host service. I didn't debug too far into things but figured I'd file an issue to raise awareness.