Page Editor shows generic 400 bad request error instead of validation error message on save

[twschiller]

Context

• In https://github.com/pixiebrix/pixiebrix-app/pull/2187 we added validation to prevent overwriting published brick versions
• In the Page Editor, this shows as receiving a generic 400 Validation error that doesn't provide any message explaining what went wrong

Implementation Sketch

• Read the messages from the config field on error JSON body and show the message in the alert toast

Workaround

• Bump the version number

Screenshot

From network inspector: