Add db-error code file

lib/db/feature-toggle-store.js

@@ -4,6 +4,7 @@ const metricsHelper = require('../metrics-helper');
 const { DB_TIME } = require('../events');
 const NotFoundError = require('../error/notfound-error');
 const FeatureHasTagError = require('../error/feature-has-tag-error');
+const { UNIQUE_CONSTRAINT_VIOLATION } = require('../error/db-error');
 
 const FEATURE_COLUMNS = [
     'name',
@@ -242,7 +243,7 @@ class FeatureToggleStore {
         await this.db(FEATURE_TAG_TABLE)
             .insert(this.featureAndTagToRow(featureName, tag))
             .catch(err => {
-                if (err.code === '23505') {
+                if (err.code === UNIQUE_CONSTRAINT_VIOLATION) {
                     throw new FeatureHasTagError(
                         `${featureName} already had the tag: [${tag.type}:${tag.value}]`,
                     );

lib/error/db-error.js

@@ -0,0 +1,3 @@
+module.exports = {
+    UNIQUE_CONSTRAINT_VIOLATION: '23505',
+};