Make raise facades macros

[vyzo]

Adds macros raise/context and defraise/context to aid capturing exception contexts, and uses them in the stdlib.
So that we have exception context everywhere!

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[vyzo]

Obligatory demo:

Gerbil 0.17.0-293-g70222be9 on Gambit v4.9.5-40-g24201248
> (import :std/misc/timeout)
> (make-timeout "foo")
*** ERROR IN (stdin)@2.1 -- 
*** ERROR IN "misc/timeout.ss"@15.25 [BadArgument]: Bad argument; expected real, time or #f
--- irritants: make-timeout "foo" 
--- continuation backtrace:
0  raise                                                                              
1  (interaction)                                                                      (stdin)@2:1                                                                        (std/misc/timeout#make-timeout "foo")

[drewc]

where: (exception-context message) is so simple yet such a great feature!!

[vyzo]

this is a sanity check, as we send the message over the wire for actor error results and a bug could end up something unintended; this is guaranteed to be a string now.