Better validation error messages

[gadomski]

Closes

• Closes Bad error message in validation #185

Description

Implements Validate for serde_json::Value, then only parse the JSON before starting validation.

New error message:

$ stac validate stac-cli/tests/data/collection-bad-temporal.json
Validation error at /extent/temporal: [["2020-12-11T22:38:32.125Z","2020-12-14T18:02:31.437Z"]] is not of type "object"
ERROR: validation errors

Checklist

Delete any checklist items that do not apply (e.g. if your change is minor, it may not require documentation updates).

• Git history is linear
• Commit messages are descriptive
• (optional) Git commit messages follow conventional commits
• Code is formatted (cargo fmt)
• cargo test
• Changes are added to the CHANGELOG