API: Proper error types instead of String

[rniii]

Work in progress. I am also taking time to unify all error types, since there were many SdlErrors before. I would like to discuss if compatibility is necessary, e.g. it could be a feature-flag that does type SdlError = String;

See also: #1053

Summary:

• Removed OpenUrlError, PrefPathError, ShowMessageError, WindowBuildError and FontError in favor of Error::InvalidString
• Removed common::IntegerOrSdlError in favor of Error::IntOverflow
• Removed set_error_from_code (internal SDL api, shouldn't be here? could add back as SdlErrorCode)
• Removed usage of NulError to Error::InvalidString?
• Fixed pref_path checking for empty string instead of null

TODO:

• Figure out TargetRenderError, TextureValueError, UpdateTextureError, UpdateTextureYUVError, AddMappingError
• ttf::InitError could be removed, if same ref-counting as subsystems is implemented for it
• ttf::FontError was removed, it might be good to review removing RendererableText, as a CString is always allocated anyway?
• if compatibility is needed, ideally we would do String -> SdlError, however this is messy because RWOps should use Error, as it currently converts io::Error to String, and that should be avoided

[rniii]

I think I noted everything down, I'll leave actually changing all strings when I'm given the green light!

It is a breaking change, however migrating code should be pretty easy: Result<T, String>s should be changed to Result<T, sdl2::Error>, and custom error types (which from the thread people have been doing) should be updated too