Ensure busy state is reset after errors

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #1518 into master will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1518      +/-   ##
==========================================
- Coverage   85.68%   85.66%   -0.03%     
==========================================
  Files         144      144              
  Lines       15944    15951       +7     
==========================================
+ Hits        13662    13664       +2     
- Misses       2282     2287       +5     

Impacted Files	Coverage Δ
panel/io/callbacks.py	29.41% <14.28%> (-1.70%)	⬇️
panel/reactive.py	85.36% <100.00%> (+0.05%)	⬆️

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