Issue title: ITexture:IsError() doesnt work properly
 Issue body: http://wiki.garrysmod.com/page/ITexture/IsError I dont think this is working as intended
https://i.imgur.com/tDUH2o0.png

As you can clearly see in my example basetexture2 shader is a a missing texture yet IsError returns false.
 Comments: 
 Comment 0: I don't think undefined texture should be error, seems more like an issue with GetName. Assuming it is undefined.
 Comment 1: Is basetexture2 undefined? My guess is undefined getTexture returns a valid texture pointing to the error texture. An errored texture would be one pointing to an invalid name.
 Comment 2: Probably easily fixed by setting the 'error' flag or whatever on that returned texture.
 Comment 3: Well, it is not broken.

It tests for the internal error flag, which is set for textures that are attempted to be loaded from dist and failed, but it is not set for the error texture itself. Setting the flag on the error texture may have unexpected results.

Also if the texture returned by GetTexture() is an error texture no value will be returned instead.
 Comment 4: well on the C++ side there's a `IsErrorTexture` function with the following expression: `return !pTex || pTex->IsError()`
There is also a `SetErrorTexture` method in ITexture with the comment: `// Set that this texture should return true for the call "IsError"`

In contrast to that there's a `IsErrorMaterial` function for IMaterial with the following expression: `return !pMat || pMat->IsErrorMaterial()`
where `IsErrorMaterial` has the following comment: `// Returns true if this is the error material you get back from IMaterialSystem38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5indMaterial if the material can't be found.`


exposing those is probably the smartest approach to handle it
 Comment 5: There is no such thing as `SetErrorTexture` in our codebase and the `IsErrorTexture` is pointless, as it will just call `IsError` which we already have bound.

`IMaterial.IsError` already calls to `IsErrorMaterial`.

I have added `ITexture.IsErrorTexture`
{'source': 'GithubIssues', 'repo_name_with_owner': 'Facepunch/garrysmod-issues', 'issue_id': 394965101, 'repo_stars': 125.0}