Return an Error on PROXY_HOST is an URL

[trobanga]

Closes #57

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.45%. Comparing base (e1f31b2) to head (4c17cb5).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4007      +/-   ##
==========================================
- Coverage   96.45%   96.45%   -0.01%     
==========================================
  Files         437      437              
  Lines       27417    27417              
  Branches      629      627       -2     
==========================================
- Hits        26446    26445       -1     
- Misses        467      469       +2     
+ Partials      504      503       -1     

see 3 files with indirect coverage changes

🚀 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.