Expected Behavior

When self.form is indexed with a key that isn't present, BadRequestKeyError is raised. Under 1.0.2 and below, the .args property of this exception contained the actual missing key -- great for lightweight error presentation.

Minimal demonstration:

from flask import Flask, jsonify, request
from werkzeug.exceptions import BadRequestKeyError
app = Flask('test')
@app.errorhandler(BadRequestKeyError)
def bad_key(e):
    return jsonify({'missing key(s)': e.args})
@app.route('/')
def root():
    return request.form['foo']
app.run()

Under 1.0.2 and below we see the following behavior:

 % curl localhost:5000
{"missing key(s)":["foo"]}

Under 1.0.3:

% curl localhost:5000
{"missing key(s)":[]}

This occurs independent of $FLASK_DEBUG

Actual Behavior

There appears to be no way to get at the missing key from an error handler.

Environment

• Python version: 3.6.6
• Flask version: 1.0.3
• Werkzeug version: 0.15.4