PageTemplate/BlockTemplate/EmailWrapper return errors instead of
panicking so hosts can compile artifact-supplied templates at load time
(codeless themes) without a malformed artifact taking down the process;
Must* variants delegate. The 'lines' filter splits a string into
trimmed non-empty lines — pongo2 string literals cannot express \n, so
multi-line textarea fields (per-line <div> addresses) were previously
inexpressible in templates.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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