Check for source in metadata when reading documents for Q&A base prompt. Fixes #2944

[cnhhoang850]

#2944

Q&A base prompt classmethod from_llm use EXAMPLE_PROMPT as document_prompt, which requires source metadata for docs retrived from vectorstore. This add check for metadata to raise an error, asking the user to ingest data with source in metadata.

[dev2049]

Let me know what you think of #3026

[hwchase17]

closing since other pr merged in