Ignore individual options for out copy

This addresses #296 by adding a fault-tolerant option to `OutputChain`. With this, it will only raise an exception if all configured outputs raise exceptions. Also added the `ignore_individual_errors` option to `out_copy` which simply changes how it configures its `OutputChain`.

@repeatedly I'll still implement your idea of `buffer_ignore_exceeded_chunk`, but in a different pull-request, is that ok?