[BUG] Runtime error when viewer is called with URL which does not exist (regression)

[stweil]

Description

The viewer needs the URL of an existing METS file.
If it is given a wrong URL, it simply did not show a digitized object in the past.
Now it shows a TYPO3 oops.

Reproduction

Steps to reproduce the behaviour:

1. Run the viewer with a URL which points to a valid METS file.
2. The viewer shows the related digitized object.
3. Run the viewer with an invalid URL.
4. The viewer shows a TYPO3 oops.

Expected Behavior

Ideally there should be a clear error message that shows which problems exist with the URL,
but at least there should not be a runtime error ("oops").

Environment

• OS version: Debian GNU Linux stable
• TYPO3 version: 11
• Release: 5.0.7