Grabbing error mesages from Authlib and passing to flash in Flask

Question

How can I pass authlib.integrations.base_client.errors.OAuthError error messages back to the html in flask via the flash() method?

I have these methods in my class

class NPIListApi:
    def __init__(self, token=None):
        self.token = token

    def get_user_token(self, un, pw):
        """generates user token so we can create a client"""
        client = OAuth2Session(c.CLIENT_ID, c.CLIENT_SECRET)
        return client.fetch_token(
            c.AUTH_URL,
            authorization_response=c.AUTH_URL,
            username=un,
            password=pw,
            grant_type="password",
        )

    def return_client(self, token):
        """creates the client required to establish connection"""
        client = OAuth2Session(
            c.CLIENT_ID,
            c.CLIENT_SECRET,
            token=token,
        )
        return client

    def establish_connection(self):
        """Method to establish connection to the LIFE API"""
        return self.return_client(self.token)

And this is my login route

@app.route("/", methods=["GET", "POST"])
def login():
    """login route where we take the users un & pw to create and store the auth token in our session"""

    form = LoginForm()

    if form.validate_on_submit():
        username = form.username.data
        password = form.password.data

        n = NPIListApi()

        # storing the token in the session not to expose un & pw
        session["token"] = n.get_user_token(username, password)
        print(session.get("token", None))

        if n.get_user_token(username, password):
            log.info(f"user: {username} logged in succesfully")
            return redirect(url_for("home"))

    return render_template("login.html", form=form)

I want to be able to flash() the error message to the user so they know if their creds are wrong etc etc.

I have tried a lot of ways but because I am returning the token in establish_connection(self) I am not sure how to do it as.

Also not how to grab the actual error its not like it's json.