Extensions to Core.Exn.
The to_string function is slightly tweaked to avoid escaping the string
content of Failure.
This is also an ever so slight variation of to_string target more at user
than developers (Failure s is just printed as s)
unwrap e
Tries to unwrap an exception to find the original cause of the error (Finally for instance has the propency to burry exception...). This is useful when matching on exceptions.