opt: better error message for multiple arbiters with CONFLICT DO UPDATE

This commit improves the error message and adds telemetry for INSERT ON
CONFLICT DO UPDATE statements that find multiple matching arbiter
indexes.

Release justification: This is a low-risk update to new functionality,
partial indexes.

Release note: None

pkg/sql/logictest/testdata/logic_test/partial_index

@@ -900,7 +900,7 @@ statement ok
 CREATE UNIQUE INDEX i2 ON u (a) WHERE b < 0
 
 # Error if the arbiter predicate does not imply the partial index predicate.
-statement error pgcode 42P10 there are multiple unique or exclusion constraints matching the ON CONFLICT specification
+statement error pgcode 0A000 there are multiple unique or exclusion constraints matching the ON CONFLICT specification
 INSERT INTO u VALUES (1, 1) ON CONFLICT (a) WHERE b < 0 AND b > 0 DO UPDATE SET b = 10
 
 statement ok

pkg/sql/opt/optbuilder/insert.go

@@ -25,6 +25,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/types"
 	"github.com/cockroachdb/cockroach/pkg/util"
+	"github.com/cockroachdb/cockroach/pkg/util/errorutil/unimplemented"
 	"github.com/cockroachdb/errors"
 )
 
@@ -830,7 +831,7 @@ func (mb *mutationBuilder) buildInputForUpsert(
 	// TODO(mgartner): Add support for multiple arbiter indexes, similar to
 	// buildInputForDoNothing.
 	if arbiterIndexes.Len() > 1 {
-		panic(pgerror.Newf(pgcode.InvalidColumnReference,
+		panic(unimplemented.NewWithIssue(53170,
 			"there are multiple unique or exclusion constraints matching the ON CONFLICT specification"))
 	}
 

pkg/sql/opt/optbuilder/testdata/upsert

@@ -2381,7 +2381,7 @@ DROP INDEX u4
 build
 INSERT INTO unique_partial_indexes VALUES (1, 1, 'bar') ON CONFLICT (b) WHERE c = 'foo' AND c = 'bar' DO UPDATE SET b = 10
 ----
-error (42P10): there are multiple unique or exclusion constraints matching the ON CONFLICT specification
+error (0A000): unimplemented: there are multiple unique or exclusion constraints matching the ON CONFLICT specification
 
 build
 INSERT INTO unique_partial_indexes VALUES (1, 1, 'bar') ON CONFLICT (b) WHERE c = 'foo' DO UPDATE SET b = 10