peer+htlcswitch: update Enable/DisableAdds API

In this commit, the `ChannelUpdateHandler`'s `EnableAdds` and
`DisableAdds` methods are adjusted to return booleans instead of errors.
This is done becuase currently, any error returned by these methods is
treated by just logging the error since today all it means is that the
proposed update has already been done. And so all we do today is log the
error. But in future, if these methods are updated to return actual
errors that need to be handled, then we might forget to handle them
correctly at the various call sights. So we instead change the signature
of the function to just return a boolean. In future, if we do need to
return any error, we will have to go inspect every call sight in any
case to fix compliation & then we can be sure we are handling the errors
correctly.

htlcswitch/mock.go

@@ -906,13 +906,14 @@ func (f *mockChannelLink) UpdateShortChanID() (lnwire.ShortChannelID, error) {
 	return f.shortChanID, nil
 }
 
-func (f *mockChannelLink) EnableAdds(linkDirection LinkDirection) error {
+func (f *mockChannelLink) EnableAdds(linkDirection LinkDirection) bool {
 	// TODO(proofofkeags): Implement
-	return nil
+	return true
 }
-func (f *mockChannelLink) DisableAdds(linkDirection LinkDirection) error {
+
+func (f *mockChannelLink) DisableAdds(linkDirection LinkDirection) bool {
 	// TODO(proofofkeags): Implement
-	return nil
+	return true
 }
 func (f *mockChannelLink) IsFlushing(linkDirection LinkDirection) bool {
 	// TODO(proofofkeags): Implement