`rustup update self` should suggest `rustup self update`

[ChrisDenton]

Problem you are trying to solve

A new-ish user got confused about the command for updating rustup itself and tried rustup update self which gave this error:

error: error: invalid value 'self' for '[toolchain]...': invalid toolchain name: 'self'

For more information, try '--help'.
: invalid toolchain name: 'self'

Solution you'd like

Maybe something like:

note: use `rustup self update` to update rustup itself

Notes

I'm also not sure why the : invalid toolchain name: 'self' is repeated at the end. That seems redundant.

### Tasks
- [ ] https://github.com/rust-lang/rustup/issues/3905

[rami3l]

@ChrisDenton Thanks a lot! Should be an easy fix.

[fbrv]

I'd like to give a shot

[rami3l]

I'd like to give a shot

@fbrv Please, go ahead!

[fbrv]

@rami3l, @djc I have something but before opening the PR I'm wondering how we want to show the message. Should we append, prepend or replace the message with the note?

[rami3l]

@rami3l, @djc I have something but before opening the PR I'm wondering how we want to show the message. Should we append, prepend or replace the message with the note?

@fbrv Following the example of rustc error messages, it seems to me that any "smart guess" lines should come after the original error message.

Please note that there could be a toolchain named self, so it's important to show the line on error exclusively.

[rami3l]

Unfortunately, @fbrv just DM'd me and expressed their wish of backing out due to a personal incident. Thanks anyway for having offered to help!

[Xerxes-2]

Could I try this?