Registry update failures should throw or return status

[omus]

When using Pkg.Registry.update() if a registry fails to be updated an error is logged but there is no exception or return value to indicate this. This can make it difficult to work with this function call in automated scripts such as Dockerfiles where you may want to abort immediately if registries fail to update:

...
#14 1.239     Updating registry at `/usr/local/share/julia-depot/registries/Private`
#14 1.275     Updating git-repo `https://github.com/.../PrivateRegistry.git`
#14 1.621 Username for 'https://github.com' [git]:     Updating registry at `/usr/local/share/julia-depot/registries/General.toml`
#14 1.816 ┌ Error: Some registries failed to update:
#14 1.816 │     — /usr/local/share/julia-depot/registries/Private — failed to fetch from repo: failed to fetch from https://github.com/../PrivateRegistry.git, error: GitError(Code:EUSER, Class:Callback, Aborting, user cancelled credential request.)
#14 1.816 └ @ Pkg.Registry /usr/local/julia/share/julia/stdlib/v1.10/Pkg/src/Registry/Registry.jl:528
#14 2.026 ┌ Warning: The active manifest file has dependencies that were resolved with a different julia version (1.11.5). Unexpected behavior may occur.
#14 2.026 └ @ /project/Manifest.toml:0
...

[DilumAluthge]

It looks like the manual doesn't currently say what the behavior of the function will be if the update fails.

So I wonder if we could make it throw, and call that a non-breaking change.

[DilumAluthge]

Technically, we'd be introducing an exception where there wasn't one before, but given that we didn't promise (in the manual) that the function wouldn't throw, maybe it's okay?