Adds two methods to result_try's exception handles:
- as_inner: returns a {l,r}-value reference either to the exception
container, or the exception_ptr. This allows to use them in operations
which work on both types, e.g. logging.
- clone_inner: returns a copy of the underlying exception container or
exception ptr.
Currently, the catch handlers in result_futurize_try are required to
return a future, although they are always being called with
seastar::futurize_invoke, so if their result is not future it could be
converted to one anyway. This commit relaxes the ConvertsWithTo
constraint in order to allow this conversion.
Adds result_try and result_futurize_try - functions which allow to
convert existing try..catch blocks into a version which handles C++
exceptions, failed results with exception containers and, depending on
the function variant, exceptional futures.
