errors: adjust ShardingError

[muzarski]

Ref: #1170

The only breaking change in this PR is unpubbing ShardingError. Remaining commits adjust the internal usages of this error - for example logging in case parsing of sharding info fails.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• [ ] I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: 44cae1f

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev bb824b61cefb8426cc00aef563ce27882cf0aa7c
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev bb824b61cefb8426cc00aef563ce27882cf0aa7c
     Cloning bb824b61cefb8426cc00aef563ce27882cf0aa7c
    Building scylla v0.15.0 (current)
       Built [  24.106s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.052s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.480s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.051s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.113s] 107 checks: 106 pass, 1 fail, 0 warn, 0 skip

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/enum_missing.ron

Failed in:
  enum scylla::routing::ShardingError, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-bb824b61cefb8426cc00aef563ce27882cf0aa7c/8e88ff90ea17547a103649e6571db48be24ce469/scylla/src/routing/sharding.rs:99

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  48.008s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  10.816s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.035s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.987s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.035s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.110s] 107 checks: 107 pass, 0 skip
     Summary no semver update required
    Finished [  22.503s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1