Create a _RETURN_IF() fpp-macro

[tclune]

A common pattern is to return early from a procedure if a certain condition holds. E.g.,

if (cond) return

Unfortunately, this does not play well with our return code mechanism for error handling, which forces us to write instead:

if (cond) then
   _RETURN(_SUCCESS)
end if

I propose a new macro _RETURN_IF(...) that returns this to a 1-liner while retaining clarity.