Behaviour is inconsistent when a command or feature is not supported #733
Description
This is something that cropped up on #695
The behaviour when a command or feature is not supported for the current buffer, either because no LSP servers are configured or no LSP servers support the required feature, is inconsistent, with some commands doing nothing and others writing a warning message (some might write an error message, I haven't tried them all).
It would be good if this behaviour was consistent, I lean towards consistently writing a warning message to tell a user an action could not be completed due to no LSP servers or no LSP servers supporting the required feature (nails two design principles, consistency and feedback). In the case of an unexpected error though, still write an error.
cc @Konfekt