Make asset manager more resiliant to assembly and type load exceptions

Summary: This adds try-catch blocks in various areas to prevent invalid / missing assemblies or types from crashing the asset manager when it's loading all of the available assets and editors in.

Test Plan: Ran the asset manager and it no longer crashed.

Reviewers: Sabre070

Reviewed By: Sabre070

Differential Revision: http://code.redpointsoftware.com.au/D118