Embedded error marker mesh

When we can't find a mesh, we display marker_error.nif (or an alternative, e.g. Collada, if that's there instead). If we can't find that error marker, either, we crash.

Better alternatives include:

• Checking for the error marker mesh and attempting to parse it on startup. If this fails, bail with a descriptive error message.
• Embed an error marker mesh in the engine somehow (e.g. an OSGT in a string). We could use the example suite's error marker for this as discussed here example-suite#37 (comment 663285761)