Display more detailed error when project fails to load

[SheepTester]

See

scratch-gui/src/lib/sb-file-uploader-hoc.jsx

Lines 160 to 163 in ba76db7

	.catch(error => {
	log.warn(error);
	alert(this.props.intl.formatMessage(messages.loadError)); // eslint-disable-line no-alert
	})

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