fix: Unwind from local stack if ucontext fails

[Swatinem]

Not quite sure if we should do this in the backend, or rather patch the unwinder itself…

fixes #384

[codecov-commenter]

Codecov Report

Merging #387 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
+ Coverage   87.49%   87.53%   +0.03%     
==========================================
  Files          49       49              
  Lines        4142     4147       +5     
==========================================
+ Hits         3624     3630       +6     
+ Misses        518      517       -1     

[Mixaill]

When we use direct unwind, do we need to skip 1 (or more) frame to hide sentry?

[GenuineAster]

5 frames including the signal handler.

[flub]

I'm not sure if we should hide frames. If this should be done at all perhaps just collapsing them by default in the UI is probably better (which I think might already happen)?

[Swatinem]

We don’t flag frames as in_app on native (yet). But yes, I would also say we should not truncate anything on the SDK side. Maybe we will set in_app in the future at some point.