wanted: easy way to capture backtraces when unwinding from conditions

[Nikodemus Siivola]

HANDLER-CASE is more idiomatic, easier to use -- and often less troublesome than HANDLER-BIND for basic condition handling.

Esp.

  (handler-case (with-mutex (m) (foo))
    (error (e) (error e))

is an important modern idiom: don't want to hold on to the lock while handling the error. It however means that actual error context is lost.

I'm thinking we could have CONDITION-BACKTRACE, which HANDLER-CASE could automatically fill before unwinding if SB-EXT:*HANDER-CASE-BACKTRACES* (or another similar toggle) is true. Those backtraces need to be purged of all stack allocated objects, and other potentially bogus things, though. Possibly just printed frames?

This would be especially convenient for reporting errors from threads.

(Can you tell I've been living inside the JVM recently? Not being able to handle conditions without unwinding sucks, but exceptions carrying their backtraces with them is pretty nice.)