improvement: add `add_error/3` #710

Job	Run time
ash-ci / mix deps.get	22s
ash-ci / MIX_ENV=test mix.compile	34s
ash-ci / MIX_ENV=dev mix.compile	38s
ash-ci / mix hex.audit	16s
ash-ci / mix dialyzer	4m 3s
ash-ci / mix credo --strict	4s
ash-ci / mix format --check-formatted	30s
ash-ci / mix sobelow	35s
ash-ci / mix spark.cheat_sheets --check	28s
ash-ci / mix spark.formatter --check	6s
ash-ci / mix test	1m 14s
ash-ci / mix deps.unlock --check-unused	41s
ash-ci / Release	0s
	9m 31s

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