Realizes the existing FIXME in ParsleyProxyElement. Replace the hardcoded if (e instanceof ParsleyUnknownKeyException) with a registry mapping exception type → disposition (inline / full-page) + a message/template generator.
Notes:
- Inline only makes sense mid-appendToResponse; take-values/invoke-action can't render inline regardless, so INLINE is a no-op outside append.
- The commented-out messageForGenericException becomes one registered generator.
Realizes the existing FIXME in ParsleyProxyElement. Replace the hardcoded
if (e instanceof ParsleyUnknownKeyException)with a registry mapping exception type → disposition (inline / full-page) + a message/template generator.Notes: