pgsql: Convert enum_in() to report errors softly.

Convert enum_in() to report errors softly.

I missed this in my initial survey, probably because I examined
the contents of pg_type in the postgres database, which lacks
any enumerated types.

Discussion: https://postgr.es/m/CAAJ_b97KeDWUdpTKGOaFYPv0OicjOu6EW+QYWj-Ywrgj_aEy1g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/442e25d2485b1ecf3357725acee995a1c349163d

Modified Files
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src/backend/utils/adt/enum.c       | 12 +++++++++---
src/test/regress/expected/enum.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/enum.sql      |  6 ++++++
3 files changed, 40 insertions(+), 3 deletions(-)