Change Chrome 4xx Page

Question 1

You can customize the error page with webNavigation API. Add an event Listener for onErrorOccurred event and update relevant details.

Check sample code as a reference.

Demonstration

manifest.json

Registered background page and added all relevant permissions to manifest file.

{
    "name": "Customize error page",
    "description": "",
    "version": "1",
    "manifest_version": 2,
    "background": {
        "scripts": [
            "background.js"
        ]
    },
    "permissions": [
        "webNavigation",
        "<all_urls>"
    ],
    "web_accessible_resources": [
        "page.html"
    ]
}

background.js

Redirected to our custom Page in case of any error, you can customize this to desired level.

//Adding a Listener to Error Occured Event
chrome.webNavigation.onErrorOccurred.addListener(function (details) {
    // Updating the browser window with desired URL
    chrome.tabs.update(details.tabId, {
        url: chrome.extension.getURL("page.html")
    });
});

page.html

Some trivial code

<html>
    <style>
        body {
            background:yellow;
            position:absolute;
        }
    </style>

    <body>
        <div style="top: 200px;position: absolute;left: 500px;width: 500;font-size: 40px;">This is a Nice Description</div>
    </body>

</html>

Reference

webNavigation.onErrorOccurred

Question 2

Go to options, privacy settings, and uncheck the box:

"Use a web service to help resolve navigation errors"

Detailed instructions here.