Fix error message and use a more fitting error code

internal/database/migrations/0014_raise_unique_constraint_error.sql

@@ -0,0 +1,13 @@
+-- +goose Up
+
+-- +goose StatementBegin
+CREATE OR REPLACE FUNCTION unique_team_slug() RETURNS trigger AS $unique_team_slug$
+    BEGIN
+        IF (SELECT slug from team_slugs WHERE slug = NEW.slug) IS NOT NULL THEN
+            RAISE 'Team slug is not available: %', NEW.slug
+            USING ERRCODE = 'unique_violation';
+        END IF;
+        RETURN NEW;
+    END;
+$unique_team_slug$ LANGUAGE plpgsql;
+-- +goose StatementEnd

internal/graph/teams.resolvers.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/google/uuid"
 	pgx "github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/nais/api/internal/auditlogger"
 	"github.com/nais/api/internal/auditlogger/audittype"
 	"github.com/nais/api/internal/auth/authz"
@@ -47,6 +48,10 @@ func (r *mutationResolver) CreateTeam(ctx context.Context, input model.CreateTea
 	err = r.database.Transaction(ctx, func(ctx context.Context, dbtx database.Database) error {
 		team, err = dbtx.CreateTeam(ctx, input.Slug, input.Purpose, input.SlackChannel)
 		if err != nil {
+			var pgErr *pgconn.PgError
+			if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+				return apierror.Errorf("Team slug %q is not available.", input.Slug)
+			}
 			return err
 		}