Way to handle the file extension error rather than just rendering the build template

[pranav1698]

I'm submitting a ... (check one with "x")

• [x] bug report
• [ ] feature request
• [ ] support request => Please do not submit support requests here, instead ask your query in out Gitter channel at https://gitter.im/fossasia/meilix

Actual Behaviour We need a way to handle the file extension error as mentioned in #381 Either a new page or a new application error handler In the current build we just render the build template instead of informing the user about the error

Steps to reproduce it Follow the gif mentioned in #381

Other information:

System information:

Ubuntu 18.04

Would you like to work on the issue? Yes, but this should be open for reviews @tabesin @meets2tarun @abishekvashok @rpotter12 please share your reviews

[pranav1698]

@tabesin please review this issue