RF: Supply end message from Builder rather than relying on JS default

[TEParsons]

In draft as this needs the accompanying change in PsychoJS (otherwise the end message becomes "Thank you for your patience." twice)

[codecov]

Codecov Report

Merging #6258 (950be89) into dev (b742eb1) will increase coverage by 0.00%.
Report is 5 commits behind head on dev.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6258   +/-   ##
=======================================
  Coverage   51.79%   51.79%           
=======================================
  Files         334      334           
  Lines       61665    61666    +1     
=======================================
+ Hits        31940    31941    +1     
  Misses      29725    29725           

Components	Coverage Δ
app	∅ <ø> (∅)
boilerplate	∅ <ø> (∅)
library	∅ <ø> (∅)
vm-safe library	∅ <ø> (∅)