Print a nicer error message when asked to build an empty string ('').

This happens sometimes, for example, if you do
	whatever | while read x; do
		redo-ifchange "$x"
	done
and the input contains blank lines.

We could ignore the request for blankness, but it seems like that
situation might indicate a more serious bug in your parser, so it's
probably better to just abort with a meaningful error.

redo-whichdo.py

@@ -12,6 +12,10 @@ if len(sys.argv[1:]) != 1:
     sys.exit(1)
 
 want = sys.argv[1]
+if not want:
+    err('cannot build the empty target ("").\n')
+    sys.exit(204)
+
 abswant = os.path.abspath(want)
 for dodir,dofile,basedir,basename,ext in paths.possible_do_files(abswant):
     dopath = os.path.join('/', dodir, dofile)