Add a more generic check_raises

[samoht]

The current one uses a strict equality, but in some cases you really want to pattern match on the exception to decide whether the function raised something in the right family of errors.

[MathiasMalandain]

I would like to "up" this issue. Being able to match something like Failure _ or Sys_error _ in a test would be great, particularly as the string literal is not something to be matched on, even according to the StdLib documentation.