Consistently clean up state after exceptions

[philippjfr]

There were still cases where Exceptions would break a panel irrecoverably.

[codecov-io]

Codecov Report

Merging #128 into master will increase coverage by 0.24%.
The diff coverage is 55.55%.

@@           Coverage Diff            @@
##           master   #128      +/-   ##
========================================
+ Coverage   89.76%    90%   +0.24%     
========================================
  Files          27     27              
  Lines        4025   4124      +99     
========================================
+ Hits         3613   3712      +99     
  Misses        412    412

Impacted Files	Coverage Δ
panel/interact.py	71.7% <0%> (ø)	⬆️
panel/param.py	94.17% <50%> (+3.9%)	⬆️
panel/viewable.py	69.19% <57.14%> (-1.13%)	⬇️
panel/pane.py	81.53% <71.42%> (-0.42%)	⬇️

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