errors: rename RequestError and make it private

[muzarski]

Ref: #519

Motivation

Current RequestError is a low-level (connection layer) error that represents a failure of ANY CQL request. Before this PR it was public. Why? There was only one place it was used in public API - namely BrokenConnectionErrorKind::KeepaliveQueryError. This is a very rare error.

I decided to make this error type pub(crate) for multiple reasons:

• As mentioned previously - this is a very rare low-level error kind. I don't think there is a point in exposing it to the user
• Thanks to that, we can rename it to InternalRequestError (since it becomes an error used only in driver's internals)
• RequestError name is now available :). It can be used for some more meaningful error returned from user-facing API. For example, an error representing a definite request failure that occurs in a session layer after possible retries, speculative execution etc.

I've hidden this type in BrokenConnectionErrorKind::KeepaliveRequestError (variant got renamed as well) behind an Arc. It's OK to do so, because this specific case is very rare. I think that not providing any additional context (i.e., ignoring the underlying error cause) is a valid alternative solution.

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: f480b0e

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 7ac4d4556053b335bf0d6439a1ac128545dc00b8
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 7ac4d4556053b335bf0d6439a1ac128545dc00b8
     Cloning 7ac4d4556053b335bf0d6439a1ac128545dc00b8
    Building scylla v0.15.0 (current)
       Built [  22.752s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.051s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.605s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.049s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.105s] 107 checks: 105 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/v0.38.0/src/lints/enum_missing.ron

Failed in:
  enum scylla::errors::RequestError, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-7ac4d4556053b335bf0d6439a1ac128545dc00b8/693dee0d209335f6347c6207669a6b08fbe5fbb8/scylla/src/errors.rs:972

--- 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/v0.38.0/src/lints/enum_variant_missing.ron

Failed in:
  variant BrokenConnectionErrorKind::KeepaliveQueryError, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-7ac4d4556053b335bf0d6439a1ac128545dc00b8/693dee0d209335f6347c6207669a6b08fbe5fbb8/scylla/src/errors.rs:836

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  46.658s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.128s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.034s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  11.035s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.033s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.107s] 107 checks: 107 pass, 0 skip
     Summary no semver update required
    Finished [  22.994s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1