Fixes for message sign and verify

base64 can throw which caused silent verification failures.
Ask for passwords with correct parent window otherwise dialog
not visible on MacOSX.  Similarly for verification messages.

gui/qt/main_window.py

@@ -183,6 +183,13 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         return "%s/%s" % (PrintError.diagnostic_name(self),
                           self.wallet.basename() if self.wallet else "None")
 
+    def top_level_window(self, window=None):
+        window = window or self
+        for n, child in enumerate(window.children()):
+            if isinstance(child, WindowModalDialog):
+                return self.top_level_window(child)
+        return window
+
     def is_hidden(self):
         return self.isMinimized() or self.isHidden()
 
@@ -1133,7 +1140,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         return value of the wrapped function, or None if cancelled.
         '''
         def request_password(self, *args, **kwargs):
-            parent = kwargs.get('parent', self)
+            parent = kwargs.get('parent', self.top_level_window())
             password = None
             while self.wallet.use_encryption:
                 password = self.password_dialog(parent=parent)
@@ -2065,21 +2072,24 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
     def do_sign(self, address, message, signature, password):
         message = unicode(message.toPlainText())
         message = message.encode('utf-8')
-        try:
-            sig = self.wallet.sign_message(str(address.text()), message, password)
-            sig = base64.b64encode(sig)
-            signature.setText(sig)
-        except Exception as e:
-            self.show_message(str(e))
+        sig = self.wallet.sign_message(str(address.text()), message, password)
+        sig = base64.b64encode(sig)
+        signature.setText(sig)
 
     def do_verify(self, address, message, signature):
         message = unicode(message.toPlainText())
         message = message.encode('utf-8')
-        sig = base64.b64decode(str(signature.toPlainText()))
-        if bitcoin.verify_message(address.text(), sig, message):
-            self.show_message(_("Signature verified"))
+        try:
+            # This can throw on invalid base64
+            sig = base64.b64decode(str(signature.toPlainText()))
+            verified = bitcoin.verify_message(address.text(), sig, message)
+        except:
+            verified = False
+        dialog = self.top_level_window()
+        if verified:
+            dialog.show_message(_("Signature verified"))
         else:
-            self.show_message(_("Error: wrong signature"))
+            dialog.show_error(_("Wrong signature"))
 
 
     def sign_verify_message(self, address=''):