Provide more info in case of infinite error nesting.

 * Added an EMERGENCY-BEST-EFFORT keyword argument to PRINT-BACKTRACE.
   The debugger calls PRINT-BACKTRACE with this flag enabled iff
   it gets nested. This way e.g. printer errors from printing
   the backtrace also land in a (nested) debugger, but only when
   coming from the first level of the debugger, otherwise they are
   muffled.

 * Make ERROR-ERROR try to print more info on the way down into
   the abyss and use this new keyword to muffle errors from
   backtrace printing.