patch to make backtrace behave better in case of infinite nested errors
a pull request on github:
Added an EMERGENCY-
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.