errors: rename and extract TracingError

[muzarski]

Ref: #519

It was previously called TracingProtocolError and was contained in ExecutionError::ProtocolError.

Since we now have a big picture of errors hierarchy, it's much clearer that TracingError should be a separate error type returned by tracing methods. ExecutionError should NOT depend on it.

This PR renames TracingProtocolError to TracingError, and changes the return type of Session::get_tracing_info(). New variant is introduced to TracingError - it contains an ExecutionError, which makes sense since we try to execute queries to system tracing tables.

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: a676f32

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 699e3895ea9fde34fb250e5581207ceee53dd2aa
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 699e3895ea9fde34fb250e5581207ceee53dd2aa
     Cloning 699e3895ea9fde34fb250e5581207ceee53dd2aa
    Building scylla v0.15.0 (current)
       Built [  21.936s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.049s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.230s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.047s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.118s] 127 checks: 125 pass, 2 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/v0.39.0/src/lints/enum_missing.ron

Failed in:
  enum scylla::errors::TracingProtocolError, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-699e3895ea9fde34fb250e5581207ceee53dd2aa/4eda4016a3500346a2fafc3e0edc84d78aee3428/scylla/src/errors.rs:267

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It 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/v0.39.0/src/lints/enum_variant_missing.ron

Failed in:
  variant ProtocolError::Tracing, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-699e3895ea9fde34fb250e5581207ceee53dd2aa/4eda4016a3500346a2fafc3e0edc84d78aee3428/scylla/src/errors.rs:216

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  45.349s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.335s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.027s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.938s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.026s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.112s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  23.313s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[wprzytula]

@muzarski a rebase is needed.

[muzarski]

Rebased on main

[muzarski]

Rebased on main again (ssl CI fix, and removed legacy ser/deser api)