[zk-token-sdk] Refactor AuthenticatedEncryptionError and ElGamalError to errors module

[samkim-crypto]

Problem

Currently, error types AuthenticatedEncryptionError and ElGamalError lives in the encryption module, which is excluded from sbf target. However, it would be nice to use these types to handle errors in the zk_token_elgamal module as well (e.g. #588), which should be included for sbf target.

Summary of Changes

Refactor these error types into the errors module so that both encryption and zk_token_elgamal submodules can access it.

Fixes #

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 3.12500% with 31 lines in your changes are missing coverage. Please review.

Project coverage is 81.8%. Comparing base (dcc195e) to head (5807079).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #589     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         850      851      +1     
  Lines      229369   229481    +112     
=========================================
+ Hits       187717   187751     +34     
- Misses      41652    41730     +78     

[joncinque]

looks good to me!

[joncinque]

nit: did cargo fmt move this down? it's up to you, but it can be more consistent to keep this block at the top, same with the other files

[samkim-crypto]

Hm, yeah it seems like cargo fmt always favors the larger chunk to be on top of the smaller chunk of imports. Alternatively, I can put an extra space between the two import chunks, but it seems like this is less clean. I'll leave it for now and maybe I can update it on a follow-up.