Better nullability and error handling

Checklist

[x] I've checked that all new and existing tests pass
[x] I've updated the documentation if necessary
[x] I've added an entry in the CHANGELOG to credit myself

Description

Fixes #290

Also does more idiomatic error handling:

Wrong:

NSError *error;
NSArray *result = [someObject someMethod error:&error];
if (!error) {
    // assume success
}

Correct:

NSError *error;
NSArray *result = [someObject someMethod error:&error];
if (result) {
    // assume success
}

Motivation and Context

The main motivation was addressing static analysis warnings I encountered when analyzing an Objective-C project. The error handling was just something I fixed along the way.

Note: I also added a couple of missing generics annotations which may affect the public interface. If they are imported into Swift, they will go from [Any] to [OHHTTPStubsDescriptor]. ~This is a breaking change for anyone who is using the output of this method in Swift.~ Actually, it may not be breaking, since in Swift, they were probably already doing something like for descriptor in descriptors as! [OHHTTPStubsDescriptor], and after this update, that line will throw a compile warning, not an error. So this can probably be a bugfix, not a major release.

Testing: I added a couple of missing tests for error throwing cases, and made sure all the tests were passing.

AliSoftware / OHHTTPStubs