I suggest you contact whoever wrote the texture pack.
The first error in your log is
Which suggests something in that texture pack is breaking the class initialisation for minecraft's texture class
Or it is conflicting with a mod that changes that code. Since you don't post a link to your debug.log we can't see which mods are modifying that code.
The log does not contain an error to explain why it can't load that class. So, we can't see the root cause.
There is this warning just before, which may or may not be relevant?
Assuming there is no fundamental problem with the resource pack (e.g. it is for a different version of minecraft)
I suspect the only way you will find the conflicting mod is to experiment with removing mods until the problem goes away.