#4075: Catch Guzzle Exception to avoid breaking harvest

[stefan-korn]

fixes #4075

• Test coverage exists
• Documentation exists

QA Steps

• Add manual QA steps in checklist format for a reviewer to perform to confirm that the feature or fix is working. Include as much details as possible so that the reviewer doesn't lose time figuring out how to perform steps.

[janette]

@stefan-korn thanks for bringing up this issue. We will want to check access to the URL before we get to the check mimetype step. And if a URL is not accessible, send a warning message to the user. So once we locate the best spot for that check (filefetcher, harvest json source validation, etc.) then we can re-work the code and tests, and merge a fix.

[paul-m]

Merging into a feature branch to expedite working on this.

Thanks @stefan-korn !

[paul-m]

Work continues: #4084