Frontend SDK: export AuthError & AuthResult

[bastienbeurier]

As mentioned in a private thread, we shoud export types for the .auth call of the Frontend SDK for a successful result:

{ providerConfigKey: string; connectionId: string }

and error responses:

{ message: string; type: string }

The auth call would go from :

nango.auth(nangoId, organization, nangoParams)
.then((result: { providerConfigKey: string; connectionId: string }) => {
    // Handle success.
.catch((err: { message: string; type: string }) => {
    // Handle error.
});

to

nango.auth(nangoId, organization, nangoParams)
.then((result: AuthResult) => {
    // Handle success.
.catch((err:  AuthError) => {
    // Handle error.
});

The developer on the same thread complains that TS compilation complaints if he doesn't guard for AuthError. He suggests we should have AuthError extend Error.