Improve error message on illegal casts

[hostilefork]

If you write:

Integer value = 10;
Float illegalFloat = static_cast<Float>(value);

The error message that comes back says:

error: call of overloaded 'Float(ren::Integer&)' is ambiguous

Really what it should say is "you can't do that". (Although whether you can do this specific case or not is up for debate in #7) Is there some static_assert or otherwise that could give a better message?

[Morwenn]

Wouldn't Float(Integer) = delete; in Float be sufficient? It would probably result in a message along the lines of "use of deleted constructor [...]" while pointing to the deleted constructor and adding a "required from here" pointing to the line where the constructor was called. If you really want to inform the user, put some comment above the deleted constructor.

[hostilefork]

I should have mentioned that it's really all illegal casts that have the lack of a good message (I just noticed this one because it was a test case of something you might think would work...) So the question was about just generally improving the message...