Validator website does not properly create ad hoc session or show any status when someone uploads a file with extension TXT

[simon-20]

Brief Description
If someone uploads a .txt file to the Validator, using the ad hoc session functionality, the website does not properly create an ad hoc session, nor show any status for that file, but simply shows the spinning "loading" icon indefinitely.

This happens whether or not the .txt is valid IATI XML.

This may be a problem for other non-xml extensions, too.

Severity
Medium / Low

Issue Location
https://validator.iatistandard.org/

Steps to Reproduce
Upload any XML file but give it a .txt extension.

Expected Results/Behaviour
The Validator website should show an appropriate error message if a non-XML text file is uploaded.

If a valid IATI XML file is uploaded with a .txt extension, the Valdiator should at the very least provide a warning.

Actual Results/Behaviour
Spinning loading logo appears indefinitely.

[tillywoodfield]

It looks like this problem would occur on any file upload error because there is no check for a successful response from the API. I've opened #635 to add a check, which causes it to show the message "File(s) uploading failed. Check your files and try again.
", but it would probably be good to show a more useful message.