Check to see if contents.xcworkspacedata exists before writing

[ollieatkinson]

Resolves #309

Short description 📝

Check for the existence of contents.xcworkspacedata before trying to write over the workspace.

Test Plan 🧪

1. Unit test to ensure we do not throw an error in the result of a failure
2. Generate a workspace for fixtures/ios_app_with_custom_workspace/, delete contents.xcworkspacedata and try to generate again

I did not deem it necessary to introduce another fixture test here, but if you feel like it is necessary I can go back and add one in.

[codecov]

Codecov Report

Merging #312 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
+ Coverage   89.31%   89.32%   +0.01%     
==========================================
  Files         276      276              
  Lines       10591    10607      +16     
==========================================
+ Hits         9459     9475      +16     
  Misses       1132     1132

Impacted Files	Coverage Δ
...ources/TuistKit/Generator/WorkspaceGenerator.swift	65.93% <ø> (ø)	⬆️
...stKitTests/Generator/WorkspaceGeneratorTests.swift	93.69% <100%> (+1.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5956b94...74ec40e. Read the comment docs.

[kwridan]

👍

[tuistbot]

	1 Warning
⚠️	Have you introduced any user-facing changes? If so, please take some time to update the documentation. Keeping the documentation up to date makes it easier for users to learn how to use Tuist.

Generated by 🚫 Danger