Ram uses word_error internally

In an effort to promote loose coupling and avoid using exceptions to pass messages, I'm going to pass them explicitly. I do suspect that reimplementing functions via enums is a bridge too far, but it allows the object and then the bus to intercept an error without me predicting who has to handle it. I'll have to reread the appropriate advice in this regard.

Oh, concretely, I mean that readMem emits a word_error struct. Ram's tests also reflect that and still pass. I began the emit/receive versions for ram, but they are placeholders until I analyze what those structs (signal/ping) need to hold.