handle bad biome files on load, instead of waiting for an IndexError later

related to <https://github.com/brownan/Minecraft-Overviewer/issues/301>

textures.py

@@ -878,14 +878,17 @@ def getBiomeData(worlddir, chunkX, chunkY):
     if biomeFile == currentBiomeFile:
         return currentBiomeData
 
-    currentBiomeFile = biomeFile
-
     f = open(os.path.join(worlddir, "biomes", biomeFile), "rb")
     rawdata = f.read()
     f.close()
-
+    
+    # make sure the file size is correct
+    if not len(rawdata) == 512 * 512 * 2:
+        raise Exception("Biome file %s is not valid." % (biomeFile,))
+    
     data = numpy.frombuffer(rawdata, dtype=numpy.dtype(">u2"))
 
+    currentBiomeFile = biomeFile
     currentBiomeData = data
     return data