Improve error message and comments

[alamb]

Rationale for this change

I am working through errors while integrating the new qualified identifiers in DataFusion and a few things are confusing:

1. What exactly normalize* was normalizing (I found the comments slightly vague)
2. When I got an error (due to a problem I am debugging), it would have helped me debug the issue more to know what the valid fields were.

What changes are included in this PR?

Changes:

1. Add some comments to Column::normalize and normalize_expr
2. Print out the available schema in error message

Are there any user-facing changes?

Previously the error message looked like this

thread 'frontend::reorg::test::test_split_plan' panicked at 'called `Result::unwrap()` on an `Err` value: Plan("No field matches column \'#t.time\'")', query/src/frontend/reorg.rs:372:78

Now the error looks like:

thread 'frontend::reorg::test::test_split_plan' panicked at 'called `Result::unwrap()` on an `Err` value: Plan("No field matches column \'#t.time\'. Available fields: foo.field_int, foo.field_int2, foo.tag1, foo.time")', query/src/frontend/reorg.rs:373:78

[houqp]

👍