Error better when provider not set

[bricelam]

I'm not sure if this was intentionally removed in v2, but we've seen a few users confused by the NullReferenceException.

Fixes #324

cc @ajcvickers

[ericsink]

Yes, I took this out on purpose, but for (what in hindsight is) a poor reason.

I'll put this (or something equivalent to it) back, but fair warning, I might not do so by simply accepting this PR.

Furthermore, I hereby acknowledge that my to-do list on SQLitePCLRaw has grown a bit lengthy, I need to devote a few days soon to catching up on everything.

[bricelam]

lol, I was going to do it a lot simpler, but decided to use the previous code.

My first implementation:

private static ISqliteProvider __imp;
private static ISqliteProvider _imp => __imp ?? throw Exception(GRIPE);

[ericsink]

Now that's rather short and sweet...