Minor database tool improvements

[mattlindsey]

Improves performance of db tool with 2 updates:

• list_tables returns a simple comma-separated list instead of an array of symbols since llms were interpreting the : at the start of each symbol as part of the table name.
• execute returns e.message when there's an exception, which helps llms retry when a query doesn't work Tested with both openai and google gemini PR.

Fixes https://github.com/patterns-ai-core/langchainrb/issues/432