Fix suggested by Sjoerd (long ago!) to get a better error message when

Fix suggested by Sjoerd (long ago!) to get a better error message when
there's a syntax error.  (In particular, display the correct
filename).  This changes the API: if there's a syntax error, the
function now returns normally after dumping the error to sys.stderr.
I changed Sjoerd's use of string.join(string.split(...)) with
string.replace().