This is a problem in the completions system since 2005f2e (Let completion functions set the
match_required state., 2009-05-08)

If the completer fails (say because it couldn't load a file and it threw an error), then the
match_required doesn't get passed through.  So, xpath webjumps will report errors properly when you try
to complete, but when you press enter it will follow a bogus url.

A kludge would be for the completer to return a special completion object in the case of error (rather
than throwing an interactive_error).  The object could have a count of zero (or null) and a "error"
member containing an error message.  It could also specify a get_match_required member as usual.  This
would allow the match_required processing to occur, so that an error will be signalled on pressing enter.
 The error member can be used to display an error from update_completions_done().

Another workaround for this specific instance would be for the handler defined in define_xpath_webjump()
to ensure that the completions are available (and that the selection is a member of the completions).

I've pushed a workaround along the lines of msg520.