commands.c: Improve error replies

commands.c: Improve error replies

- Improve / add various error messages.
- Replace all `LOG(…); ysuccess(false);` with `yerror(…);`.
- switch_mode: Remove redundant "ERROR:" ELOG string.
- cmd_move_con_to_workspace*: Make sure that we don't try to move an
empty workspace to another workspace. This can be problematic when we
match a workspace using command criteria (eg marks) and the target is a
non-existing workspace. We create the new workspace but since nothing is
moved there, we are left with an empty workspace. See added testcase.