[COMMITTERS] pgsql: Give nicer error message when connecting to a v10 serverrequiri

Give nicer error message when connecting to a v10 server requiring SCRAM.

This is just to give the user a hint that they need to upgrade, if they try
to connect to a v10 server that uses SCRAM authentication, with an older
client.

Commit to all stable branches, but not master.

Discussion: https://www.postgresql.org/message-id/bbf45d92-3896-eeb7-7399-2111d517261b@pivotal.io

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f050c847d9698e89e145c20cddb7f41535d9d121

Modified Files
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src/include/libpq/pqcomm.h     |  2 ++
src/interfaces/libpq/fe-auth.c | 13 +++++++++++++
2 files changed, 15 insertions(+)