Logg errors at the Google Scholar fetcher and hide them from the UI

[JofielB]

Summary

This pull request is related to #6334
This pull request is to resolve the exception raised at the UI when you make a search at Google Scholar.

Changes

Move the error thrown to the logger as is recommended.

Demo

No UI error through when you make a search at Google Scholar

Error show at the log as a warning

• Change in CHANGELOG.md described (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 created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

Thank you for the PR. It contains two classes, where only one is relevant for the PR title. For the other PR, throwing a new FetcherException is the "JabRef" style. Adding log output is done at #7075