Rely on convert_channel_err over locked_close_chan

While most of our closure logic mostly lives in
`locked_close_chan`, some important steps happen in
`convert_channel_err` and `handle_error` (mostly broadcasting a
channel closure `ChannelUpdate` and sending a relevant error
message to our peer).

While its fine to use `locked_close_chan` directly when we
manually write out the relevant extra steps, its nice to avoid it
to DRY up some of our closure logic, which we do here.

Author: TheBlueMatt