Fix error handling for group sessions

Fix a couple of places where we were using the wrong method to get the last
error.

javascript/olm_inbound_group_session.js

@@ -66,14 +66,14 @@ InboundGroupSession.prototype['decrypt'] = restore_stack(function(
 ) {
     var message_array = array_from_string(message);
     var message_buffer = stack(message_array);
-    var max_plaintext_length = session_method(
+    var max_plaintext_length = inbound_group_session_method(
         Module['_olm_group_decrypt_max_plaintext_length']
     )(this.ptr, message_buffer, message_array.length);
     // caculating the length destroys the input buffer.
     // So we copy the array to a new buffer
     var message_buffer = stack(message_array);
     var plaintext_buffer = stack(max_plaintext_length + NULL_BYTE_PADDING_LENGTH);
-    var plaintext_length = session_method(Module["_olm_group_decrypt"])(
+    var plaintext_length = inbound_group_session_method(Module["_olm_group_decrypt"])(
         this.ptr,
         message_buffer, message_array.length,
         plaintext_buffer, max_plaintext_length

javascript/olm_outbound_group_session.js

@@ -54,7 +54,7 @@ OutboundGroupSession.prototype['unpickle'] = restore_stack(function(key, pickle)
 });
 
 OutboundGroupSession.prototype['create'] = restore_stack(function(key) {
-    var random_length = session_method(
+    var random_length = outbound_group_session_method(
         Module['_olm_init_outbound_group_session_random_length']
     )(this.ptr);
     var random = random_stack(random_length);