Use None instead of exception if an unknown channel dtype was read

[BeardedBeaver]

If an unknown channel dtype was read from ld file, currently reader raises an exception and no channel is returned. So it's impossible to update meta_ptr and the file becomes effectively unreadable. Instead I write None as dtype indicating that it's an unknown type and throw an exception in data, so the user can read all channels and deal with unknown formats later

[gotzl]

Sounds reasonable, thanks!