Improve validation for ReactiveHTML validation

[philippjfr]

• Better error when jinja2 template rendering fails
• Ensure loop variables are uniquely named

[codecov]

Codecov Report

Merging #2332 (4ff4017) into master (a6dbf03) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2332      +/-   ##
==========================================
- Coverage   84.10%   84.08%   -0.02%     
==========================================
  Files         181      181              
  Lines       22064    22069       +5     
==========================================
  Hits        18557    18557              
- Misses       3507     3512       +5     

Impacted Files	Coverage Δ
panel/models/reactive_html.py	81.37% <50.00%> (-0.44%)	⬇️
panel/reactive.py	76.17% <50.00%> (-0.18%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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