input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL statement as input and return a Pandas DataFrame. If the connection to the database is not provided, the function should raise a ValueError.
  assert convert_sql_to_df("SELECT * FROM book;")!=None
  assert convert_sql_to_df("SELECT id, title FROM book;").shape==(10, 2)
output: def convert_sql_to_df(sql_statement):
      import pandas as pd
      import sqlite3
      try:
          conn = sqlite3.connect('example.db')
          df = pd.read_sql_query(sql_statement, conn)
          conn.close()
          return df
      except Exception as e:
          raise ValueError("Connection to the database failed")