Add RUST_BACKTRACE=1 to env when launching core

[cmyr]

• closes #371

Review Checklist

• [ ] I have responded to reviews and made changes where appropriate.
• [x] I have tested the code
• [x] I have updated comments / documentation related to the changes I made.
• [x] I have rebased my PR branch onto xi-mac/master.

[codecov-io]

Codecov Report

Merging #374 into master will decrease coverage by 15.32%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #374       +/-   ##
===========================================
- Coverage   41.59%   26.26%   -15.33%     
===========================================
  Files          36       36               
  Lines        4130     4134        +4     
===========================================
- Hits         1718     1086      -632     
- Misses       2412     3048      +636

Impacted Files	Coverage Δ
Sources/XiEditor/RPCSending.swift	50.38% <100%> (-6.6%)	:arrow_down:
Sources/XiEditor/XiTextPlane/Atlas.swift	10.71% <0%> (-66.08%)	:arrow_down:
Sources/XiEditor/XiTextPlane/TextLine.swift	0% <0%> (-49.62%)	:arrow_down:
Sources/XiEditor/Fps.swift	11.39% <0%> (-44.31%)	:arrow_down:
Sources/XiEditor/EditView.swift	16.04% <0%> (-38.4%)	:arrow_down:
Sources/XiEditor/XiTextPlane/Renderer.swift	50.25% <0%> (-36.05%)	:arrow_down:
Sources/XiEditor/StyleMap.swift	0% <0%> (-34.4%)	:arrow_down:
Sources/XiEditor/LineCache.swift	7.76% <0%> (-32.04%)	:arrow_down:
Sources/XiEditor/ClientImplementation.swift	8.69% <0%> (-30.44%)	:arrow_down:
Sources/XiEditor/Document.swift	44.85% <0%> (-11.22%)	:arrow_down:
... and 6 more

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[jansol]

Wouldn't an if let have an empty body and do everything in the else clause? Unless you meant using the ?? operator in which case I agree that would look nicer.