Fix some panics related to BigInt operations

[georgeroman]

This should fix the panics from #883. However, I'm not very sure if this is the right way to do it or if there is a better way to generate errors instead of panicking, so I would appreciate some feedback on it.

[codecov]

Codecov Report

Merging #884 into master will increase coverage by 0.13%.
The diff coverage is 63.33%.

@@            Coverage Diff             @@
##           master     #884      +/-   ##
==========================================
+ Coverage   59.30%   59.43%   +0.13%     
==========================================
  Files         165      165              
  Lines       10342    10399      +57     
==========================================
+ Hits         6133     6181      +48     
- Misses       4209     4218       +9     

Impacted Files	Coverage Δ
boa/src/value/operations.rs	47.74% <40.00%> (+1.00%)	⬆️
boa/src/builtins/bigint/mod.rs	70.14% <84.61%> (+6.81%)	⬆️
boa/src/builtins/bigint/operations.rs	85.71% <88.23%> (+30.87%)	⬆️
boa/src/object/gcobject.rs	65.60% <0.00%> (-7.64%)	⬇️
boa/src/builtins/object/mod.rs	64.00% <0.00%> (-0.87%)	⬇️
boa/src/builtins/json/mod.rs	82.05% <0.00%> (-0.85%)	⬇️
boa/src/object/mod.rs	46.45% <0.00%> (-0.54%)	⬇️
boa/src/value/display.rs	88.50% <0.00%> (-0.14%)	⬇️
boa/src/context.rs	61.64% <0.00%> (+0.55%)	⬆️
boa/src/value/mod.rs	74.58% <0.00%> (+0.80%)	⬆️
... and 6 more

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[Razican]

Looks very good, thanks!