Abort if the cluster exists when clustering

[fredfrance-oss]

While the issue #271 mentions a warning, there seems to be no reason to warn when doing an incorrect merge. This patch bails if the output cluster directory exists.

In the unlikely case that cl1 does not exist, but cl2 does, cl1 will be created and then left as we bail. Solving this unlikely scenario would require an initial check-exists-loop which does not really make sense.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 66.56%. Comparing base (55fe2dc) to head (b973a0e).

Files with missing lines	Patch %	Lines
casr/src/util.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
+ Coverage   66.53%   66.56%   +0.02%     
==========================================
  Files          34       34              
  Lines        7591     7599       +8     
==========================================
+ Hits         5051     5058       +7     
- Misses       2540     2541       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.