Check if online file before downloading link

[btut]

When importing, JabRef offers an option to download online files. This PR adds a check that files are indeed online links before downloading.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: 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 to the documentation repository.

[Siedlerchr]

lgtm. Anything else you want to do because it's marked as draft?

[btut]

Issue was reported by @koppor, would like to wait for him to confirm it is fixed. Sorry, should have put that in the description.

[koppor]

The PDF was https://link.springer.com/book/10.1007/978-3-319-47590-5.

Step 1:

Step 2:

Works now. However, I should not use that way in the concrete case. I should merely import the DOI and then download the PDF.