Error types everywhere instead of String

[fluffysquirrels]

As discussed in issue #942.

[fluffysquirrels]

Is there anything I can do to help move this along?

[Cobrand]

I'm sorry, I terribly misunderstood what you meant in the other issue, and unfortunately I think this PR is a breaking change only for a breaking change. Changing String to an enum with the only variant being String is a bit of a waste... If you added some variants, that would probably be worth the while, and that's what I thought you would do when we discussed in the other issue, but here, I don't think it's a necessary breaking change.

If you want your error type to implement Error, what we can do is create a tuple struct SdlErrorWrapper(String) which implements Error so it can be used by other libraries more easily... although this would only save a dozen lines at most, no more.