wf: don't crash if graphviz is not installed

wcs/admin/workflows.ptl

@@ -134,8 +134,11 @@ def graphviz(workflow, url_prefix='', select=None, svg=True,
     print >>out, '}'
     out = out.getvalue()
     if svg:
-        process = Popen(['dot', '-Tsvg', '/dev/stdin'], stdin=PIPE, stdout=PIPE)
-        out, err = process.communicate(out)
+        try:
+            process = Popen(['dot', '-Tsvg', '/dev/stdin'], stdin=PIPE, stdout=PIPE)
+            out, err = process.communicate(out)
+        except OSError:
+            return ''
         if include:
             out = graphviz_post_treatment(out)
             # It seems webkit refuse to accept SVG when using its proper namespace,