Error Migration and Refactoring for 3.6.0

[StandingPadAnimations]

This PR is mainly to migrate errors in MCprep to use the new MCprepError class. At some point, we realized the need for being able to return objects with messages, so we also added an optional msg field to MCprepError.

In this PR, we also perform any general refactoring that may crop up as needed. Most of these are related to error handling, but there are a few that for the sake of getting out of the way I've done, which are:

• Removal of Blender Internal and Blender 2.7X code in world_tools.py
• Removal of 2.7X link and append operations in bAppendLink, as well as deprecate the active_layer argument

The following functions have been refactored to use the new error type:

• convert_mtl: Optional[bool] -> Optional[MCprepError]
• enable_obj_importer: Optional[Bool] -> Union[OBJImportCode, MCprepError]
• bAppendLink: bool -> Optional[MCprepError]
• open_program: Union[int, str] -> Optional[MCprepError]

[StandingPadAnimations]

I think I'll wait till #555 is merged before continuing to work on this, since both #555 and this PR have changes in world_tools.py. One #555 is merged, I'll update this branch with minestone-3-6-0 and continue working on this

[StandingPadAnimations]

I think we should merge this into the CommonMCOBJ branch so we can knock out 2 birds with one stone

[StandingPadAnimations]

Alright, merging into the CommonMCOBJ branch