Display more detailed error when project fails to load

[SheepTester]

See

https://github.com/SheepTester/scratch-gui/blob/ba76db7350bd43b79119cac2701bc10f6c511f0c/src/lib/sb-file-uploader-hoc.jsx#L160-L163

Sometimes projects fail to load for mysterious reasons, and it'd help me debug these issues if I could see the error in the alert pop-up without having to tell them to open the console

Maybe it could say

The project file that was selected failed to load.

Technical details:

Error: Non-ascii character in FixedAsciiString
  at B (lib.min.js:165)
  at t.read (lib.min.js:165)
  at t.get [as version] (lib.min.js:165)
  at t.value (lib.min.js:165)
  at t.value (lib.min.js:165)
  at new A (lib.min.js:165)
  at lib.min.js:136