Adds support for custom error handling.

[brycecovert]

This adopts the pattern of clj-cbor for clj-pgp. It doesn't apply the pattern everywhere, but allows users to have partial success when there is a single error in a pgp stream.

Couple notes:
Unlike clj-cbor, I decided to support a cause, to allow deeper exception context to be supported if the consumer wants to surface that.

[codecov-io]

Codecov Report

Merging #22 into develop will decrease coverage by 0.03%.
The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           develop      #22      +/-   ##
===========================================
- Coverage    94.12%   94.09%   -0.03%     
===========================================
  Files            7        8       +1     
  Lines          579      593      +14     
  Branches         9        9              
===========================================
+ Hits           545      558      +13     
- Misses          25       26       +1     
  Partials         9        9

Impacted Files	Coverage Δ
src/clj_pgp/core.clj	96.09% <100%> (+0.26%)	⬆️
src/clj_pgp/error.clj	83.33% <83.33%> (ø)

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[greglook]

A few thoughts.

[greglook]

A few small things to fix, then this will be good to go.

[greglook]

Thanks!