Add a SQL agent for interacting with SQL Databases and JSON Agent for interacting with large JSON blobs (#1150)

This PR adds 

* `ZeroShotAgent.as_sql_agent`, which returns an agent for interacting
with a sql database. This builds off of `SQLDatabaseChain`. The main
advantages are 1) answering general questions about the db, 2) access to
a tool for double checking queries, and 3) recovering from errors
* `ZeroShotAgent.as_json_agent` which returns an agent for interacting
with json blobs.
* Several examples in notebooks

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/sql_database.py

@@ -5,7 +5,7 @@ from typing import Any, Iterable, List, Optional
 
 from sqlalchemy import MetaData, create_engine, inspect, select
 from sqlalchemy.engine import Engine
-from sqlalchemy.exc import ProgrammingError
+from sqlalchemy.exc import ProgrammingError, SQLAlchemyError
 from sqlalchemy.schema import CreateTable
 
 
@@ -187,3 +187,33 @@ class SQLDatabase:
                     raise ValueError("Fetch parameter must be either 'one' or 'all'")
                 return str(result)
         return ""
+
+    def get_table_info_no_throw(self, table_names: Optional[List[str]] = None) -> str:
+        """Get information about specified tables.
+
+        Follows best practices as specified in: Rajkumar et al, 2022
+        (https://arxiv.org/abs/2204.00498)
+
+        If `sample_rows_in_table_info`, the specified number of sample rows will be
+        appended to each table description. This can increase performance as
+        demonstrated in the paper.
+        """
+        try:
+            return self.get_table_info(table_names)
+        except ValueError as e:
+            """Format the error message"""
+            return f"Error: {e}"
+
+    def run_no_throw(self, command: str, fetch: str = "all") -> str:
+        """Execute a SQL command and return a string representing the results.
+
+        If the statement returns rows, a string of the results is returned.
+        If the statement returns no rows, an empty string is returned.
+
+        If the statement throws an error, the error message is returned.
+        """
+        try:
+            return self.run(command, fetch)
+        except SQLAlchemyError as e:
+            """Format the error message"""
+            return f"Error: {e}"