inferior: Distinguish inferior exceptions.

inferior: Distinguish inferior exceptions.

This avoids ambiguities when looking at a backtrace where the exception
was actually thrown by an inferior in a very different context.

* guix/inferior.scm (&inferior-exception): New condition type.
(read-repl-response): Add optional 'inferior' parameter.  Raise
'&inferior-exception' instead of rethrowing to KEY when receiving an
'exception' message.
(read-inferior-response): Pass INFERIOR to 'read-repl-response'.
* tests/inferior.scm ("&inferior-exception"): New test.