Improve verbosity of auth error

src/auth/AuthGuard.ts

@@ -47,7 +47,7 @@ export default abstract class AuthGuard<P extends AuthProof> {
                 throw new Error('Unable to register user.');
             }
         } else if (registerCallback) {
-            throw new AuthError('User already exists.');
+            throw new UserAlreadyExistsAuthError(await proof.getEmail());
         }
 
         session.auth_id = user.id;
@@ -84,3 +84,12 @@ export class AuthError extends Error {
         super(message);
     }
 }
+
+export class UserAlreadyExistsAuthError extends AuthError {
+    public readonly email: string;
+
+    constructor(userEmail: string) {
+        super(`User with email ${userEmail} already exists.`);
+        this.email = userEmail;
+    }
+}