warn user if no Ghostscript on their machine

This is the thread which motivated me to write this:
http://gramps.1791082.n4.nabble.com/Re-Relationship-graph-is-limited-to-1x1-page-td4680666.html
but unfortunately the Nabble archive of gramps was dead at
the start of that thread, so it is missing.

Here is a related commit, addressing part of the problem:
90de264ccf

gramps/gen/plug/docgen/graphdoc.py

@@ -8,7 +8,7 @@
 # Copyright (C) 2007       Brian G. Matherly
 # Copyright (C) 2009       Benny Malengier
 # Copyright (C) 2009       Gary Burton
-# Copyright (C) 2017       Mindaugas Baranauskas
+# Copyright (C) 2013       Mindaugas Baranauskas <embar@super.lt>
 # Copyright (C) 2017       Paul Culley
 #
 # This program is free software; you can redistribute it and/or modify
@@ -911,6 +911,15 @@ class GVPdfGsDoc(GVDocBase):
         options.menu.get_option_by_name('dpi').set_value(72)
         GVDocBase.__init__(self, options, paper_style)
 
+    def init(self):
+        """
+        tell the user how to enable this output format, if necessary
+        """
+        if _GS_CMD == '':
+            from gramps.gen.errors import ReportError
+            raise ReportError(_("'Ghostscript' is not installed"),
+                              _("Use your package manager to install it"))
+
     def close(self):
         """ Implements GVPdfGsDoc.close() """
         GVDocBase.close(self)
@@ -1007,7 +1016,6 @@ FORMATS = []
 
 if _DOT_FOUND:
 
-    if _GS_CMD != "":
     FORMATS += [{'type' : "gspdf",
                  'ext'  : "pdf",
                  'descr': _("PDF (Ghostscript)"),