Need better sig-fail diagnostics by default. "warning: Signature verification failed" is insufficient.

When we verify the signature, we 'correctly' simply report success/failure, however it may be of value to report more detail by default so that if users encounter the issue seen by the OP, we can more easily diagnose the problem as often these things are transient and hard to reproduce.

ORIGINAL ISSUE

Problem
I just ran rustup update locally, and I received a warning that "signature verification failed" when downloading what appears to be the manifest for the nightly channel versions. This is the warning I received:

warning: Signature verification failed for 'https://static.rust-lang.org/dist/channel-rust-nightly.toml'

And here is the entire output from running rustup update:

info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
warning: Signature verification failed for 'https://static.rust-lang.org/dist/channel-rust-nightly.toml'
info: latest update on 2020-08-18, rust version 1.47.0-nightly (792c645ca 2020-08-17)
info: downloading component 'rustfmt'
info: downloading component 'rust-src'
info: downloading component 'rust-std'
info: downloading component 'rustc'
 51.5 MiB /  51.5 MiB (100 %)  32.6 MiB/s in  1s ETA:  0s
info: downloading component 'clippy'
info: downloading component 'cargo'
info: removing previous version of component 'rustfmt'
info: removing previous version of component 'rust-src'
info: removing previous version of component 'rust-std'
info: removing previous version of component 'rustc'
info: removing previous version of component 'clippy'
info: removing previous version of component 'cargo'
info: installing component 'rustfmt'
info: Defaulting to 500.0 MiB unpack ram
info: installing component 'rust-src'
info: installing component 'rust-std'
 20.7 MiB /  20.7 MiB (100 %)  13.4 MiB/s in  1s ETA:  0s
info: installing component 'rustc'
 51.5 MiB /  51.5 MiB (100 %)  14.7 MiB/s in  3s ETA:  0s
info: installing component 'clippy'
info: installing component 'cargo'

  stable-x86_64-unknown-linux-gnu unchanged - rustc 1.45.2 (d3fb005a3 2020-07-31)
   nightly-x86_64-unknown-linux-gnu updated - rustc 1.47.0-nightly (792c645ca 2020-08-17) (from rustc 1.47.0-nightly (7e6d6e5f5 2020-08-16))

info: cleaning up downloads & tmp directories

Steps
I unfortunately have not able to reproduce this bug. Running rustup update again did not give the same warning, nor did removing the nightly toolchain and installing from scratch again. I also tried moving my ~/.rustup directory to somewhere else and running rustup instlal nightly again, but I didn't get the warning that time either. I'm a little hesitant to completely uninstall rustup and install everything from scratch again, so I figured I would wait until I heard back on this issue to see if that would be useful in some way.

Notes

This occurred on an Arch Linux box with rustup installed through the package manager.

Output of rustup --version:

rustup 1.22.1 (2020-07-08)

Output of rustup show:

Default host: x86_64-unknown-linux-gnu
rustup home:  /home/saghm/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu (default)
nightly-x86_64-unknown-linux-gnu
1.43.1-x86_64-unknown-linux-gnu

installed targets for active toolchain
--------------------------------------

x86_64-unknown-linux-gnu
x86_64-unknown-linux-musl

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (default)
rustc 1.45.2 (d3fb005a3 2020-07-31)

Start with the signature verification path used by rustup update and the code that emits the Signature verification failed warning. Determine which verification details are available when the check fails, expose useful diagnostics by default, and confirm that failed and successful updates report the intended information.