Return an error if a single-row mutation does not affect exactly one row.

[jmalloc]

Fixes #100

Also provides better context in the face of duplicate errors and general improvements to MySQL error handling.

[codecov-io]

Codecov Report

Merging #108 into master will decrease coverage by 0.13%. The diff coverage is 2.4%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   30.38%   30.24%   -0.14%     
==========================================
  Files          65       65              
  Lines        2156     2169      +13     
==========================================
+ Hits          655      656       +1     
- Misses       1491     1503      +12     
  Partials       10       10

Impacted Files	Coverage Δ
src/axmysql/saga/snapshot.go	0% <0%> (ø)	:arrow_up:
src/axmysql/saga/crud.go	0% <0%> (ø)	:arrow_up:
src/axmysql/messagestore/store.go	0% <0%> (ø)	:arrow_up:
src/ax/projection/consumer.go	0% <0%> (ø)	:arrow_up:
src/axmysql/messagestore/append.go	0% <0%> (ø)	:arrow_up:
src/axmysql/saga/keyset.go	0% <0%> (ø)	:arrow_up:
src/axmysql/outbox/repository.go	87.15% <100%> (+0.11%)	:arrow_up:

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[koden-km]

Review approved.