Previously, any error reported by the backend while reading
system_constraints.sql would report the entire file, not just the
particular command it was working on.  (Ask me how I know.)  Likewise,
there were chunks of system_functions.sql that would be read as one
command, which would be annoying if anything failed there.

The issue for system_constraints.sql is an oversight in commit
dfb75e47.  I didn't try to trace down where the poor formatting
in system_functions.sql started, but it's certainly contrary to
the advice at the head of that file.