Convert exception to string first; Fixes #2157

CHANGELOG

@@ -1,5 +1,8 @@
 #Nmap Changelog ($Id$); -*-text-*-
 
+o [Zenmap][GH#2157] Fixed an issue where a failure to execute Nmap would result
+  in a Zenmap crash with "TypeError: coercing to Unicode" exception.
+
 o Nmap no longer considers an ICMP Host Unreachable as confirmation that a
   target is down, in accordance with RFC 1122 which says these errors may be
   transient. Instead, the probe will be destroyed and other probes used to

zenmap/zenmapGUI/ScanInterface.py

@@ -513,7 +513,7 @@ class ScanInterface(HIGVBox):
         except Exception, e:
             warn_dialog = HIGAlertDialog(
                 message_format=_("Error executing command"),
-                secondary_text=unicode(e, errors='replace'),
+                secondary_text=unicode(str(e), errors='replace'),
                 type=gtk.MESSAGE_ERROR)
             warn_dialog.run()
             warn_dialog.destroy()