Give a useful message about PYTHONPATH if startup fails

hg

@@ -1,20 +1,27
 #!/usr/bin/env python
 #
 # mercurial - scalable distributed SCM
 #
 # Copyright 2005-2007 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
 # enable importing on demand to reduce startup time
-from mercurial import demandimport; demandimport.enable()
+try:
+    from mercurial import demandimport; demandimport.enable()
+except ImportError:
+    import sys
+    sys.stderr.write("abort: couldn't find mercurial libraries in [%s]\n" %
+                     ' '.join(sys.path))
+    sys.stderr.write("(check your install and PYTHONPATH)\n")
+    sys.exit(-1)
 
 import sys
 import mercurial.util
 import mercurial.dispatch
 
 for fp in (sys.stdin, sys.stdout, sys.stderr):
     mercurial.util.set_binary(fp)
 
 mercurial.dispatch.run()