Convert old latin1 labels, Raise exception if json conversion fails.

A couple of changes
1) Old electrum wallets seemed to save labels in latin1, when you call json.dumps on line 83/84 it fails silently, which causes the label import to fail. Whenever electrum saves, it then overwrites your default wallet with no labels - essentially deleting all your labels. The solution to this is iterating over all the labels for old wallets decoding anything that fails to unicode() as latin1, and then unicoding it :)

2) Failing to import data and then deleting it is bad. So I'm raising an exception to avoid data being lost.

lib/wallet.py

@@ -72,12 +72,18 @@ class WalletStorage(object):
             except Exception as e:
                 raise IOError("Cannot read wallet file '%s'" % self.path)
             self.data = {}
+            # In old versions of Electrum labels were latin1 encoded, this fixes breakage.
+            for i, label in d['labels'].items():
+                try:
+                    unicode(label)
+                except UnicodeDecodeError:
+                    d['labels'][i] = unicode(label.decode('latin1'))
             for key, value in d.items():
                 try:
                     json.dumps(key)
                     json.dumps(value)
                 except:
-                    continue
+                    raise Exception('Failed to convert wallet to 2.x format')
                 self.data[key] = value
         self.file_exists = True