Added the `error' module.

This makes a global error type which we can use conveniently with
`setjmp()' based error handling.
Added `setjmp()' based error handling to `grafix'.
Added `asset' module and IAM loader.

IAM stands for `Isomyr Asset Manifest', it just lists a bunch of
assets, their types, and where to find them.
Added a test for the asset database loader.