Provide more info in case of infinite error nesting.

 * 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.