fix(server): replace email-in-use messages to prevent user-existence oracle

Error messages on registration and profile-update that named whether an
email address was already taken allowed callers to enumerate registered
accounts. All three sites now return the same generic message regardless
of whether the address is in use.

e2e/src/specs/server/api/user.e2e-spec.ts

@@ -120,7 +120,7 @@ describe('/users', () => {
         .set('Authorization', `Bearer ${nonAdmin.accessToken}`);
 
       expect(status).toBe(400);
-      expect(body).toMatchObject(errorDto.badRequest('Email already in use by another account'));
+      expect(body).toMatchObject(errorDto.badRequest('Email is not available'));
     });
 
     it('should update my email', async () => {