add standard exit code for non-payable functions

[alexytsu]

Adds a standard exit code that allows actors to indicate they are not prepared to receive funds: filecoin-project/builtin-actors#836

Example usage in builtin-actors: filecoin-project/builtin-actors#1241

[codecov-commenter]

Codecov Report

Merging #1696 (3499a68) into master (7c44970) will increase coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1696   +/-   ##
=======================================
  Coverage   55.42%   55.43%           
=======================================
  Files         146      146           
  Lines       14031    14031           
=======================================
+ Hits         7777     7778    +1     
+ Misses       6254     6253    -1     

Impacted Files	Coverage Δ
shared/src/error/mod.rs	0.00% <ø> (ø)
ipld/kamt/src/bitfield.rs	87.89% <0.00%> (+0.63%)	⬆️

[anorth]

This works for me. Let's wait for @Stebalien as he suggested it.

[Stebalien]

It works for me too. My only reservations are:

1. We didn't reserve enough of these...
2. FEVM actors won't use this exit code unless we liberally apply some magic sauce.

But I think we should have this.