Check for missing assemblies on startup

[Reousa]

Description

Currently DR2 does not check for missing assemblies that are required, but have not been loaded for any reason - mainly DLLs not existing in the DR2 plugin folder.
In reference to #97

Possible fix

One could check what assemblies are required, and then check whether they physically exist in DR2's probing directories or not, throwing readable errors when they don't.

That should give users an easier time debugging these issues, as they're currently hard to figure out for users that are not familiar with DR2 or don't have source access.