Standardized (web) application error messages? [closed]

1. How do I handle server error in '/' application?
2. What is the most common error code?

How do I handle server error in '/' application?

Update the URL

When this is the case, the error description in the browser will clarify that the error has occurred because the file is missing or has been renamed. The “Server error in '/' application” can occur as an http 404 error. To fix this problem, correct the URL in the link that triggers this error.

What is the most common error code?

404 Not Found

The most common error code you run into is a 404 error. The 404 status code means the requested resource is no longer available or, more specifically, just not found. Was it ever available there?