Made FluentError init method public

[calebkleveter]

No description provided.

[rafiki270]

@calebkleveter in my case I just needed this error only in tests so I used @testable import Fluent to get it. Disregard my comment if you need it on the front

[calebkleveter]

I could use that hack, but I was going to use it in model inits:

guard let id = model.id else {
    throw FluentError(identifier: "noID", reason: "A model must have an ID to be connected to a pivot"
}

[tanner0101]

Agreed, this will be nice for the fluent drivers as well.