Fix unnecessary re_viewer dependency

[Wumpf]

What

The mechanism for identifying Rerun clients was implemented in #6204. However, during the release I accidentally "fixed" the build of cargo build -p re_viewer --no-default-features twice:

• The seemingly correct fix was to depend on re_sdk_comms with the server feature always enabled.
• The definitely wrong thing that slipped through is to feature guard the section that needs the re_sdk_comms server errors with #[cfg(feature = "server")]. But re_viewer doesn't have a server feature, disabling this effectively. This was reverted in Don't log warnings when unknown clients connect over TCP #6368

It's up to users of re_viewer to allow the serve feature, making re_viewer agonistic of the presence of this error type in its smart channel. Therefore I now moved out the error type to re_sdk_comms lib.rs so that re_viewer doesn't have to depend on the re_sdk_comms's server feature.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
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[Wumpf]

this is really only a difference to #6368 if you somehow manage to depend on re_viewer without depending re_sdk_comms = { workspace = true, features = ["server"] }. Which as of now never happens as far as I can tell

[Wumpf]

corrections: the wasm blob itself shouldn't drag in re_sdk_comms with server enabled

• it doesn't break anything functionally but may make it larger than needed (could be all stripped though)
• this breaks cargo clippy --quiet --all-features --target wasm32-unknown-unknown --target-dir target_wasm -p re_viewer -- --deny warnings, making nightly ci break https://github.com/rerun-io/rerun/actions/runs/9125203480/job/25090909224