Wrap the body of sb-debug:backtrace with with-debug-io-syntax. Added with-debug-io-syntax macro.
Provide more info in debugger-disabled-hook.

Before this change if there was an error printing the condition object,
then we didn't even try printing the backtrace afterwards, which can be
a useful source of information even if the condition printing has failed.
Make the printing of a slot-unbound error more error tolerant.

Especially against errors coming from custom PRINT-OBJECT methods, in which
case only print the TYPE-OF the instance. Also, print fully qualified
symbol names.
Use sb-impl::print-symbol-with-prefix in implicit-generic-function-warning.