wmllint: fixed an error when trying to check if a file is a savegame

The error was caused by the fact that opening a file with codecs.open() is much stricter than using open(mode=rb), and throws a UnicodeDecodeError if a file can't be read as UTF-8 - which is the case for binary files.

data/tools/wesnoth/wmltools.py

@@ -199,8 +199,12 @@ def issave(filename):
         with gzip.open(filename) as content:
             firstline = content.readline()
     else:
-        with codecs.open(filename, "r", "utf8") as content:
-            firstline = content.readline()
+        try:
+            with codecs.open(filename, "r", "utf8") as content:
+                firstline = content.readline()
+        except UnicodeDecodeError:
+            # our saves are in UTF-8, so this file shouldn't be one
+            return False
     return firstline.startswith("label=")
 
 def isresource(filename):