Show a nicer error message in Ndiff when an input file can't be opened.

CHANGELOG

@@ -1,5 +1,8 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [Ndiff] Show a nicer error message when an input file can't be
+  loaded. Suggested by Derril Lucci, who also contributed a patch.
+
 o Added a function that was missing from http-favicon.nse. Its absence
   would cause the error
     http-favicon.nse:141: variable 'dirname' is not declared

ndiff/ndiff

@@ -1193,10 +1193,14 @@ def main():
     filename_a = input_filenames[0]
     filename_b = input_filenames[1]
 
-    scan_a = Scan()
-    scan_a.load_from_file(filename_a)
-    scan_b = Scan()
-    scan_b.load_from_file(filename_b)
+    try:
+        scan_a = Scan()
+        scan_a.load_from_file(filename_a)
+        scan_b = Scan()
+        scan_b.load_from_file(filename_b)
+    except Exception, e:
+        print >> sys.stderr, u"Can't open file: %s" % str(e)
+        sys.exit(EXIT_ERROR)
 
     diff = ScanDiff(scan_a, scan_b)