Fix blank screen for frontend errors

[jzwang43]

Reasons for making this change

Blank screen show after edit source and then click save.

This is because error is an object and cannot be directly rendered. Because the error message is just error 404, I added clearer message to explain the error to user.

Related issues

#3651

Screenshots

Checklist

• I've added a screenshot of the changes, if this is a frontend change
• I've added and/or updated tests, if this is a backend change
• I've run the pre-commit.sh script
• I've updated docs, if needed

[epicfaace]

This seems fine, but does the error object already contain a message that we could show to the user? or is it better to hard-code the error message as we do in this PR?

[jzwang43]

This seems fine, but does the error object already contain a message that we could show to the user? or is it better to hard-code the error message as we do in this PR?

The error.message just says something like "error 404" so I think it's better to give some more user-friendly error message.