AO3-6084 Improve error behavior for Add Prompt form (prompt memes)

[techlingyi]

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-6084

Purpose

Add a potential error message section to the Add Prompt form. When the user uses the Add Prompt form to add a prompt with a noncanonical tag, instead of redirecting to the Edit Sign-up form, just re-render the Add Prompt form (preserving the user's input); the noncanonical tag error message (already set up for Edit Sign-up and Edit Prompt) automatically appears in the potential error message section.

(n.b. I didn't add any extra tests, just edited the ones that covered the original behavior)

Credit

Ling-Yi (any pronouns)