Improve NotFound exception message to include namespace

[jacobtomlinson]

While investigating dask/dask-kubernetes#903 it felt useful to know which namespace kr8s is looking in for a resource, but the NotFound exception doesn't show this. This PR improves the error message to include the namespace.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.08%. Comparing base (87063fc) to head (4c63e24).
Report is 138 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/_objects.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
+ Coverage   94.61%   95.08%   +0.46%     
==========================================
  Files          29       30       +1     
  Lines        3141     3889     +748     
==========================================
+ Hits         2972     3698     +726     
- Misses        169      191      +22     

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

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests