Provide more info in case of infinite error nesting (was needed to debug a swank error)

 * Added an EMERGENCY-BEST-EFFORT keyword argument to PRINT-BACKTRACE,
   disabled by default. The debugger calls PRINT-BACKTRACE with the
   flag on only when it gets nested, so that e.g. errors from printing
   the backtrace also land in a (nested) debugger, but only when
   coming from the first level of the debugger.

 * Make ERROR-ERROR try to print more info on the way down into
   nesting and use this new keyword.

regarding the first point, another option would be to enable the flag by default, but then it's much harder to ask the debugger to enter into a nested session when there are e.g. print errors in the backtrace, so i chose the option described above, but i'm open for suggestions.