This section describes exception-handling and exception-raising constructs
provided by the (rnrs exceptions (6))library.
[R6RS] Handler must be a procedure and should accept one argument.
Thunk must be a procedure that accepts zero arguments. The
with-exception-handler procedure returns the results of invoking
thunk. When an exception is raised, handler will be invoked with
the exception.
[R6RS] Each cond-clause is as in the specification of cond.
and else are the same as in the (rnrs base (6)) library.
Evaluating a guard form evaluates body with an exception handler
that binds the raised object to variable and within the scope of that
binding evaluates the clauses as if they were the clauses of a condexpression. If every cond-clause's test evaluates to #f and there is
no else clause, then raise is re-invoked on the raised object.
[R6RS] Raises a non-continuable exception by invoking the current exception handler on obj.
[R6RS] Raises a continuable exception by invoking the current exception handler on obj.