Add new rule to SwiftLint for enforcing specific ACL on Error types

[svenmuennich]

To be able log or display the correct type of an error, its ACL must be sufficiently open for the error handling to be able to see the type. Hence any Error types declared within the same module of the logging/displaying must have at least internal ACL, while any Error types declared in a module consumed by the logging/displaying module (i.e. a library) must have at least public ACL.

We should add a new rule to SwiftLint called minimum_error_acl for enforcing a minimum ACL for Error types. The rule should have a configuration option acl which can be set to any of Swift's ACLs, but defaults to public.