Make libpq error messages consistent for translation

The errormessage for an incorrect require_auth method wasn't using the
common "invalid %s value" errormessage which lessens the burden on our
translators.  Fix by changing to that format to make use of existing
translations and to make error messages consistent in wording.

Reported and fixed by Gurjeet Singh with some tweaking by myself.

Author: Gurjeet Singh <gurjeet@singh.im>
Discussion: https://postgr.es/m/CABwTF4Xu3g9zohJ9obu8m7MKbf8g63NgpRDjwqPHQgAtB+Gb8Q@mail.gmail.com

src/interfaces/libpq/fe-connect.c

@@ -1407,8 +1407,8 @@ connectOptions2(PGconn *conn)
 			else
 			{
 				conn->status = CONNECTION_BAD;
-				libpq_append_conn_error(conn, "invalid require_auth method: \"%s\"",
-										method);
+				libpq_append_conn_error(conn, "invalid %s value: \"%s\"",
+										"require_auth", method);
 
 				free(part);
 				return false;

src/test/authentication/t/001_password.pl

@@ -264,7 +264,7 @@ $node->connect_fails(
 $node->connect_fails(
 	"user=scram_role require_auth=none,abcdefg",
 	"unknown require_auth methods are rejected",
-	expected_stderr => qr/invalid require_auth method: "abcdefg"/);
+	expected_stderr => qr/invalid require_auth value: "abcdefg"/);
 
 # For plain "password" method, all users should also be able to connect.
 reset_pg_hba($node, 'all', 'all', 'password');