errors: rename QueryError to ExecutionError

[muzarski]

Ref: #519
Step forward: #713

As the PR titles says, this PR renames the QueryError to ExecutionError. I also checked the mentions of the QueryError in the docs - last commit adjusts the docs regarding the error returned in case of schema agreement timeout.

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: 39b51df

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 4c464b674bf85b733239d24968c2e8b4271ce38d
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 4c464b674bf85b733239d24968c2e8b4271ce38d
     Cloning 4c464b674bf85b733239d24968c2e8b4271ce38d
    Building scylla v0.15.0 (current)
       Built [  24.644s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.050s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  23.695s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.048s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.133s] 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::QueryError, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-4c464b674bf85b733239d24968c2e8b4271ce38d/69b61ef29a49fc1bddac0860bba2dec3848833ec/scylla/src/errors.rs:46

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn 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/inherent_method_missing.ron

Failed in:
  RequestAttemptError::into_query_error, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-4c464b674bf85b733239d24968c2e8b4271ce38d/69b61ef29a49fc1bddac0860bba2dec3848833ec/scylla/src/errors.rs:915
  PartitionKeyError::into_query_error, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-4c464b674bf85b733239d24968c2e8b4271ce38d/69b61ef29a49fc1bddac0860bba2dec3848833ec/scylla/src/statement/prepared_statement.rs:498
  PartitionKeyError::into_query_error, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-4c464b674bf85b733239d24968c2e8b4271ce38d/69b61ef29a49fc1bddac0860bba2dec3848833ec/scylla/src/statement/prepared_statement.rs:498
  RequestError::into_query_error, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-4c464b674bf85b733239d24968c2e8b4271ce38d/69b61ef29a49fc1bddac0860bba2dec3848833ec/scylla/src/errors.rs:835

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  49.524s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.588s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.035s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  11.564s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.034s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.128s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  23.999s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1