In commit 882e707e40 we changed message
output to go to stdout by default, unless it was an error. The plain()
function doesn't *look* like an error function, but in practice it was
-- it's used to continue multiline messages, and all in-tree uses were
for warning/error.
This is a problem both because we're sending output to the wrong place,
and because in some cases, we were performing error logging from a
function which would otherwise return a value to be captured in a
variable using command substution.
Fix this and straighten out the API by providing two functions: one for
continuing msg output, and one which wraps this by sending output to
stderr, for continuing error output.
Change all callers to use the second function.
bf458cced7