Improve error reporting when *CATCH-ERRORS* is :VERBOSE

* common.lisp (resignalled-condition): Use a new slot
ORIGINAL-CONDITION-BACKTRACE.
(explain-failsafe): Improve text.
(explain-failsafe unconvertible-argument): New method.
(explain-failsafe :after): Print the "...when it was bitten by"
here. Explain the original condition and the backtrace.

* safe-simple-read.lisp (print-object :snooze-reader-error): New
method.
(safe-simple-read-from-string): Check for empty string.