Reword some awkward compiler error messages

[ericlippert]

Summary: The compiler error messages were a little bit awkward:

The model uses a & operation unsupported by Bean Machine Graph.

Not wrong but hard to read. We always said a instead of an when necessary, we capitalized inconsistently, we used symbols instead of words inconsistently, and so on.

Not every error message is perfect now, but as a result of these changes (1) we use articles mostly correctly, (2) capitalization is mostly correct, (3) operators are described using both words and symbols as appropriate.

We still do not do error messages involving illegal values very well but I will rewrite those in a later diff.

Reviewed By: yucenli

Differential Revision: D34191131