runrst: improve error message when Docutils is missing

Peter Toft told me he had installed a 'python-doc' package instead of
the correct 'python-docutils' and he suggested that we add the URL to
Docutils in our error message.

doc/runrst

@@ -1,47 +1,49
 #!/usr/bin/env python
 #
 # runrst - register custom roles and run correct writer
 #
 # Copyright 2010 Matt Mackall <[e-mail]> and others
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """usage: %s WRITER args...
 
 where WRITER is the name of a Docutils writer such as 'html' or 'manpage'
 """
 
 import sys
 try:
     from docutils.parsers.rst import roles
     from docutils.core import publish_cmdline
     from docutils import nodes, utils
 except ImportError:
     sys.stderr.write("abort: couldn't generate documentation: docutils "
                      "module is missing\n")
+    sys.stderr.write("please install python-docutils or see "
+                     "http://docutils.sourceforge.net/\n")
     sys.exit(-1)
 
 def role_hg(name, rawtext, text, lineno, inliner,
             options={}, content=[]):
     text = "hg " + utils.unescape(text)
     linktext = nodes.literal(rawtext, text)
     parts = text.split()
     cmd, args = parts[1], parts[2:]
     if cmd == 'help' and args:
         cmd = args[0] # link to 'dates' for 'hg help dates'
     node = nodes.reference(rawtext, '', linktext,
                            refuri="hg.1.html#%s" % cmd)
     return [node], []
 
 roles.register_local_role("hg", role_hg)
 
 if __name__ == "__main__":
     if len(sys.argv) < 2:
         sys.stderr.write(__doc__ % sys.argv[0])
         sys.exit(1)
 
     writer = sys.argv[1]
     del sys.argv[1]
 
     publish_cmdline(writer_name=writer)