Better error messages for ops forbidden in Eager mode

[rnett]

Currently, if you try to use placeholder in Eager mode you get a cryptic error message that says nothing about it being forbidden in eager in mode (I had to look it up on StackOverflow). As far as I'm aware there's no ops that are eager only, but I think throwing a custom exception for mode-dependent ops is reasonable and helpful.

I'd probably do this in EagerOperationBuilder.build rather than in the ops classes to catch custom created ops. If that's fine I can make a PR if a list of forbidden ops is provided (or just with placeholder).