Improve CantFindColumn and ColumnAlreadyExists errors

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Description

• Changed all CantFindColumn errors to list the column name. This makes the following message more clear:

〉let b = 'c'
〉{a:1} | select $b
Error: nu::shell::column_not_found (link)

  × Cannot find column
   ╭─[entry #12:1:1]
 1 │ {a:1} | select $b
   · ──┬──          ─┬
   ·   │             ╰── cannot find column
   ·   ╰── value originates here
   ╰────

〉

The error is changed to:

  × Cannot find column
   ╭─[entry #11:1:1]
 1 │ {a:1} | select $b
   · ──┬──          ─┬
   ·   │             ╰── cannot find column 'c'
   ·   ╰── value originates here
   ╰────

• ColumnAlreadyExists errors are changed likewise.

User-Facing Changes

See above.

Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all tests pass

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.

[rgwood]

Nice change - I love to see error messages getting better. Thanks.