Allow reseting to previous error stacks

[Affonso-Gui]

Split from #362 to be easier to merge.

This is necessary for euslime.

Copy pasting description below

With this fixes, (reset 2) throws 2 and goes back to level 1:

1.eus$ (let ((a 1)) (error "1st error"))
Call Stack (max depth: 20):
  0: at (error "1st error")
  1: at (let ((a 1)) (error "1st error"))
  2: at #<compiled-code #X64dfdc0>
eus 0 error:  1st error in (error "1st error")
2.E1-eus$ (let ((b 2)) (error "2nd error"))
Call Stack (max depth: 20):
  0: at (error "2nd error")
  1: at (let ((b 2)) (error "2nd error"))
  2: at euserror
  3: at euserror
  4: at (error "1st error")
  5: at (let ((a 1)) (error "1st error"))
  6: at #<compiled-code #X64dfdc0>
eus 0 error:  2nd error in (error "2nd error")
3.E2-eus$ (eval-dynamic 'a)
1
4.E2-eus$ (eval-dynamic 'b)
2
5.E2-eus$ (reset 2)
6.E1-eus$ (eval-dynamic 'a)
1
7.E1-eus$ (eval-dynamic 'b)
*unbound*

[Affonso-Gui]

EDIT: actually this does not seems to be necessary for euslime because it overwrites the euserror

[k-okada]

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