Forms: Add error message on network request failure for ajax submission

[dhasilva]

Follow-up to #44204

Proposed changes:

• Adds an error message when form submission fails for any reason other than validation

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

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Testing instructions:

• With the feature flag enabled, go to a form
• Make sure the page is fully loaded and disable the network on the browser network monitor
• Submit the form
• Check that a message is displayed for 5 seconds

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the update/forms-submit-ajax-frontend-errors branch.
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bin/jetpack-downloader test jetpack update/forms-submit-ajax-frontend-errors

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/forms-submit-ajax-frontend-errors

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[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 4 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/modules/form/view.js	0/213 (0.00%)	0.00%	5 💔
projects/packages/forms/src/modules/form/shared.js	0/33 (0.00%)	0.00%	3 ❤️‍🩹
projects/packages/forms/src/contact-form/class-contact-form.php	600/1102 (54.45%)	-0.05%	1 ❤️‍🩹
projects/packages/forms/src/contact-form/class-contact-form-plugin.php	425/1181 (35.99%)	0.05%	0 💚

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[CGastrell]

LGTM! Works as described!

[simison]

We could specify that there was connection error to avoid being too vague, how about this:

Connection issue while submitting the form. Check that you are connected to the Internet and try again.

cc @ilonagl

[ilonagl]

Sounds great! +1 to being more clear on what the error means and how to recover.