Add fallback

[koppor]

This PR depends on #14504 (because of the used Localization string)

When invalid data is pasted, the data is collected in the comment field.

Example:

Try to paste

@Article{Kopp:2023:LTG,
  author =       "Oliver Kopp",
  title =        "The {\LaTeX} template generator: How micro-templates
                 reduce template maintenance effort",
  journal =      [j-TUGboat](http://ftp.math.utah.edu/pub/tex/bib/tugboat.html#j-TUGboat),
  volume =       "44",
  number =       "2",
  pages =        "203--207",
  year =         "2023",
  CODEN =        "????",
  DOI =          "https://doi.org/10.47397/tb/44-2/tb137kopp-microtemplates",
  ISSN =         "0896-3207",
  ISSN-L =       "0896-3207",
  bibdate =      "Mon Sep 18 10:39:32 MDT 2023",
  bibsource =    "https://www.math.utah.edu/pub/tex/bib/tugboat.bib",
  URL =          "https://tug.org/TUGboat/tb44-2/tb137kopp-microtemplates.pdf",
  acknowledgement = [ack-nhfb](http://ftp.math.utah.edu/pub/tex/bib/tugboat.html#ack-nhfb),
  fjournal =     "TUGboat",
  issue =        "137",
  journal-URL =  "https://tug.org/TUGboat/",
  ORCID-numbers = "Kopp, Oliver/0000-0001-6962-4290",
  remark =       "Intermediate Plus{\Dash}separating writing from
                 knowledge of packages, classes, tools.",
}
``´

<img width="562" height="306" alt="image" src="https://github.com/user-attachments/assets/85002ae8-1656-4b29-8f2b-11039e818f08" />

<img width="855" height="339" alt="image" src="https://github.com/user-attachments/assets/88278bb5-4456-4407-8a33-8453c58b9445" />

Next step: Support nested BibTeX entries

### Steps to test

See above

### Mandatory checks

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- [x] I own the copyright of the code submitted and I license it under the [MIT license](https://github.com/JabRef/jabref/blob/main/LICENSE)
- [x] I manually tested my changes in running JabRef (always required)
- [/] I added JUnit tests for changes (if applicable)
- [x] I added screenshots in the PR description (if change is visible to the user)
- [x] I described the change in `CHANGELOG.md` in a way that is understandable for the average user (if change is visible to the user)
- [/] I checked the [user documentation](https://docs.jabref.org/): Is the information available and up to date? If not, I created an issue at <https://github.com/JabRef/user-documentation/issues> or, even better, I submitted a pull request updating file(s) in <https://github.com/JabRef/user-documentation/tree/main/en>.

[Siedlerchr]

but in the code it's comment

[koppor]

Oh, yeah, and comment is better IMO.