Stop crashes during exception handling

[johnfairh]

The try! in RbException.description is a landmine - reliably fails if Ruby is messed up, easiest way to recreate is to remove the RUBY_INIT_STACK call. It turns out Swift itself calls description for try! via swift_UnexpectedError. We get:

1. try! require(‘set’) (or whatever)
2. Ruby fails with errinfo set
3. RbVM.doProtect() -> RbError.raise() does throw
4. Swift calls error.description
5. RbError.description -> RbExn.description -> try! call(‘class’)
6. Generates a new error, back to step 4

Fixes #55, #58

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.01%. Comparing base (2e6ea2a) to head (d0b96ad).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #60   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files          22       22           
  Lines        2141     2141           
=======================================
  Hits         2077     2077           
  Misses         64       64           

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