Update DOI error message

[larsgw]

Fixes #2719

Description

Update error message for DOIs where no metadata can be found.

Testing

• E.g. this nonexistent DOI: http://127.0.0.1:8100/doi/10.1002/pssb.202600316

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have not used code from external sources without attribution
• I have considered accessibility in my implementation
• There are no remaining debug statements (print, console.log, ...)