Added functions analogous to 'try' and 'tryJust' from Control.Exception

`Control.Exception` exports two functions, `try` and `tryJust`, whose behaviour might be easily missed when using `pipes-safe` error handling facilities.

Here two functions are added:
- `Control.Proxy.Safe.exTry`, analogous to `Control.Exception.try`.
- `Control.Proxy.Safe.exTryJust`, analogous to `Control.Exception.tryJust`.

I'm not particularly fond of their names—“ex” stands for “exception”, by the way—but at least they don't start with “try”, a prefix already used for different purposes in the same module. The name `catchEither` is arguably better, but then `catchEitherJust` is weird.

Each of these functions is added in a different commit.