More improvements for loading notebook extension

[philippjfr]

Turns out there were more problems with the notebook load script. Specifically if you got into a failed loading state, there was no way to recover because root._bokeh_is_loading would still be non-zero, indicating that initialization was still in process and should therefore be skipped. We also now do not override the existing timeout, fix the error handling when a resource is unavailable, reset the timeout when we re-attempt loading Bokeh. Lastly we avoid loading Bokeh and Panel twice if HoloViews is imported.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.19%. Comparing base (d5070c9) to head (97d45df).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/config.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7274   +/-   ##
=======================================
  Coverage   82.18%   82.19%           
=======================================
  Files         337      337           
  Lines       50364    50363    -1     
=======================================
+ Hits        41392    41394    +2     
+ Misses       8972     8969    -3     

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