errors: metadata errors refactor

[muzarski]

Ref: #519

Motivation

The goal of this PR is to purge metadata.rs of QueryError, and replace it with MetadataError. Thanks to that metadata related functions are now independent of QueryError.

I tried to split this PR into commits, so each commit handles one function/method from metadata module. New error variants are introduced where necessary.

When it comes to public API - after this PR Session::refresh_metadata() will now return MetadataError.

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: 898b1f8

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev d40e1846961f9e6116c5f719c13d4ffbb681e6da
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev d40e1846961f9e6116c5f719c13d4ffbb681e6da
     Cloning d40e1846961f9e6116c5f719c13d4ffbb681e6da
    Building scylla v0.15.0 (current)
       Built [  24.134s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.059s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.372s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.050s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.110s] 107 checks: 106 pass, 1 fail, 0 warn, 0 skip

--- 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 ProtocolError::InvalidCqlType, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-d40e1846961f9e6116c5f719c13d4ffbb681e6da/f203671dfa2c3e0c818ba852f967e3e3e1c39a50/scylla/src/errors.rs:335

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  47.808s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  10.834s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.034s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  11.129s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.034s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.108s] 107 checks: 107 pass, 0 skip
     Summary no semver update required
    Finished [  22.709s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[wprzytula]

🔧 (EDIT: I see it's mentioned in the commit message) As you make make_keyspace_filtered_query_pager a generic method, the comment is no longer valid.

❓ Is it feasible to keep the common part nongeneric and only add generics as a wrapper over it?

[wprzytula]

⛏️ I feel that inlining the pager_fut here (making the code flow linear) could be more readable here.