Improve handling of `clap` errors

[rami3l]

Problem you are trying to solve

Split from #3812:

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'

@ChrisDenton and @Xerxes-2 have expressed concerns about duplicated info in the error message.

Solution you'd like

Make the error report format cleaner by changing it to something like:

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

For more information, try '--help'.

Notes

No response