Startup improvements

[mras0]

• Fix startup bugs (VideoControlTags being called on invalid CM + CINIT not working as expected)
• Handle resource acquisition failures during loading (OOM/file not found). May still leak resources (file handles/memory in some cases though).

[mheyer32]

LGTM. If Karlos has no issues, feel free to merge. Thanks for finding those init bugs!

[mras0]

No problem, it's been fun figuring out how this project works.

Added two more minor things changes that were bugging me:

• mt_init wrote past its allocated buffer (I think due to alignment particularities this wouldn't actually cause an issue in practice, but with this change I can now load the game, start a level and exit cleanly with enforcer+debugger attached on my real amiga)
• Errors in Vid_OpenMainScreen were not being handled, in particular a wrong mode ID would just cause a hang. We can, and maybe should, argue about error handling, but for now I've made use of the newly added "Sys_FatalErrror" function.

@0xABADCAFE Sorry this is turning into a big one. I can split it up if you like, but I recommend probably checking each commit in isolation. On their own they aren't that large.

[0xABADCAFE]

I think we should merge this regardless of whether or not we think the dos.library suggestion has any legs, since that can always be a later change.