Fixed bug where replacing missing values (with `set-value` restart) was causing duplications in the output
Fixed bug where replacing missing values (with `set-value` restart)
was causing duplications in the output

Because we kept trying to rerun sub-parts of the template with more
provided values, each part up to the error would be output repeatedly
The solution is to buffer the output till we are sure the whole
sub-template ran correctly, then play the buffer into the parent sink
once we are sure of success (ie: no error skipped passed that line).

re ADWolf:#1254 (2.5)

lucashpandolfo/talcl