Add DecodeError::DangerousValue for decoding invalid channel managers

This would help distinguish different types of errors when deserialzing
a channel manager. InvalidValue was used previously but this could be
because it is an old serialization format, whereas DangerousValue is a
lot more clear on why the deserialization failed.

Author: benthecarman

lightning/src/ln/channelmanager.rs

@@ -10927,13 +10927,13 @@ where
 						log_error!(logger, " client applications must ensure that ChannelMonitor data is always available and the latest to avoid funds loss!");
 						log_error!(logger, " Without the latest ChannelMonitor we cannot continue without risking funds.");
 						log_error!(logger, " Please ensure the chain::Watch API requirements are met and file a bug report at https://github.com/lightningdevkit/rust-lightning");
-						return Err(DecodeError::InvalidValue);
+						return Err(DecodeError::DangerousValue);
 					}
 				} else {
 					// We shouldn't have persisted (or read) any unfunded channel types so none should have been
 					// created in this `channel_by_id` map.
 					debug_assert!(false);
-					return Err(DecodeError::InvalidValue);
+					return Err(DecodeError::DangerousValue);
 				}
 			}
 		}

lightning/src/ln/msgs.rs

@@ -91,6 +91,11 @@ pub enum DecodeError {
 	Io(io::ErrorKind),
 	/// The message included zlib-compressed values, which we don't support.
 	UnsupportedCompression,
+	/// Value is validly encoded but is dangerous to use.
+	/// This is used for things like [`crate::ln::channelmanager::ChannelManager`] deserialization
+	/// where we want to ensure that we don't use a [`crate::ln::channelmanager::ChannelManager`]
+	/// which is in the past.
+	DangerousValue,
 }
 
 /// An [`init`] message to be sent to or received from a peer.