Correctly report error when target dir does not exist.

If ./default.do knows how to build x/y/z, then we will run
	./default.do x/y/z x/y/z x__y__z.redo2.tmp
which can correctly generate $3, but then we can fail to rename it to
x/y/z because x/y doesn't exist.  This would previously through an
exception.  Now it prints a helpful error message.

default.do may create x/y, in which case renaming will succeed.

builder.py

@@ -244,7 +244,15 @@ class BuildJob:
             rv = 207
         if rv==0:
             if st2:
-                os.rename(self.tmpname2, t)
+                try:
+                    os.rename(self.tmpname2, t)
+                except OSError, e:
+                    dnt = os.path.dirname(t)
+                    if not os.path.exists(dnt):
+                        err('%s: target dir %r does not exist!\n' % (t, dnt))
+                    else:
+                        err('%s: rename %s: %s\n' % (t, self.tmpname2, e))
+                        raise
                 os.unlink(self.tmpname1)
             elif st1.st_size > 0:
                 try: