add ImpreciseExceptions

[ijones]

see ImpreciseExceptions.

(JL). We've had challenges with imprecise exceptions in practice. It is very easy to expect that the expression will have been evaluated (and hence the exception raised), and it wasn't. So the error is not caught. Worse, it may get evaluated later, and raise the exception outside the scope of the intended handler. *Imprecise* captures them well. They seem to be so imprecise as to be detremental.

(FJH). Having a standard deepSeq would help address most of the problems that JL is referring to, I think.