chore: add error when no new diffs generated

[liamdebeasi]

Issue number: N/A

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What is the current behavior?

When the "Update Reference Screenshot" job is run but no new screenshots are generated, the following error is logged:

nothing added to commit but untracked files present (use "git add" to track)
Error: Process completed with exit code 1.

This is happening because there are no files to commit when running git commit. Brandy noted that this was confusing since it doesn't actually tell you why there are no files to commit.

What is the new behavior?

• If there are no diffs to commit then the update screenshot script will log a message saying that no screenshot diffs were generated.

Example:

⚠️ Error: No new screenshots generated ⚠️
This means that there were zero visual diffs when running screenshot tests.
Make sure you have pushed any code changes that would result in visual diffs.

https://github.com/ionic-team/ionic-framework/actions/runs/6099399582/job/16552017414

Does this introduce a breaking change?

• Yes
• No

Other information