The problem:

Recently when I tried to add a theme by uploading a zipped theme, the import failed, and the error dialog box contained: Failed to import theme. We were unable to complete the import of this theme. As this was not enough information for me to be able to solve the problem, I had to open a ticket with Zendesk support, who were able to tell me that the error in their error log was source sequence is illegal/malformed utf-8, which meant that one of the files in the theme was encoded in an invalid way. To locate the file with the invalid encoding, I exported an existing theme, and then replaced the files in the zipped theme one at a time, attempting to import after each replacement. In this way I did locate the file, but it was a laborious process, as it was the 14th file that I replaced.

My suggestion:

When an import fails, put the specific error in the error dialog box, and also put in the name of the file where the error occurred. Additionally, the import error dialog box disappears after a few seconds, although it has a functioning close button. The dialog box should disappear only when the close button is clicked, to give the user time to take a screenshot of it.