FR: More informative error messages on mod launch

[EatThePath]

At present the error messages that you get when launching a mod are often not helpful in solving the error. Some information that would help:

• If two different conflicting versions of the same mod are being requested, list where the requirements are coming from and what they are specifically. Currently I believe it just gives the name of the mod that is in conflict.
• If a missing package is the problem, specify what package(s) are the issue in addition to the mod name. Presently it only lists the name of the mod, which is quite confusing if the user already has the mod and doesn't realize to check the packages.