Warn users if extension is not loaded before server page is rendered

[philippjfr]

Fixes #2765

[codecov]

Codecov Report

Merging #2766 (0624e23) into master (6a9e254) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2766      +/-   ##
==========================================
- Coverage   82.75%   82.73%   -0.03%     
==========================================
  Files         188      188              
  Lines       24095    24100       +5     
==========================================
- Hits        19940    19939       -1     
- Misses       4155     4161       +6     

Impacted Files	Coverage Δ
panel/pane/deckgl.py	73.18% <ø> (ø)
panel/widgets/ace.py	100.00% <ø> (ø)
panel/widgets/tables.py	85.35% <ø> (ø)
panel/widgets/terminal.py	75.00% <ø> (ø)
panel/util.py	81.52% <28.57%> (-1.27%)	⬇️
panel/pane/echarts.py	77.58% <100.00%> (ø)
panel/pane/equation.py	85.41% <100.00%> (ø)
panel/pane/perspective.py	54.14% <100.00%> (ø)
panel/pane/plotly.py	84.98% <100.00%> (ø)
panel/pane/vega.py	82.64% <100.00%> (ø)
... and 2 more

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