ux: return better error message if a segment doesn't exist (#4122)

Catch cases where the segment doesn't exist and populate that error
message with more info: it now says that a segment
with <id> doesn't exist instead of just 'No row'.

src/lib/db/segment-store.test.ts

@@ -0,0 +1,30 @@
+import { ISegmentStore } from '../types/stores/segment-store';
+import dbInit from '../../test/e2e/helpers/database-init';
+import getLogger from '../../test/fixtures/no-logger';
+import NotFoundError from '../error/notfound-error';
+
+let stores;
+let db;
+let segmentStore: ISegmentStore;
+
+beforeAll(async () => {
+    db = await dbInit('segment_store_serial', getLogger);
+    stores = db.stores;
+    segmentStore = stores.segmentStore;
+});
+
+afterAll(async () => {
+    await db.destroy();
+});
+
+describe('unexpected input handling for get segment', () => {
+    test("gives a NotFoundError with the ID of the segment if it doesn't exist", async () => {
+        const id = 123;
+        try {
+            await segmentStore.get(id);
+        } catch (e) {
+            expect(e instanceof NotFoundError).toBeTruthy();
+            expect(e.message).toEqual(expect.stringMatching(id.toString()));
+        }
+    });
+});

src/lib/db/segment-store.ts

@@ -195,7 +195,12 @@ export default class SegmentStore implements ISegmentStore {
             .from(T.segments)
             .where({ id });
 
-        return this.mapRow(rows[0]);
+        const row = rows[0];
+        if (!row) {
+            throw new NotFoundError(`No segment exists with ID "${id}"`);
+        }
+
+        return this.mapRow(row);
     }
 
     async addToStrategy(id: number, strategyId: string): Promise<void> {