input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL query and a database connection as input and return a Pandas DataFrame. If the SQL query is invalid or the connection fails, the function should return an error message.
  assert convert_sql_to_dataframe("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn))
  assert convert_sql_to_dataframe("SELECT title, author FROM book;", conn).equals(pd.read_sql_query("SELECT title, author FROM book;", conn))
  assert convert_sql_to_dataframe("", conn)=="Invalid SQL query"
  assert convert_sql_to_dataframe("SELECT * FROM nonexistent_table;", conn)=="Error executing query"
output: def convert_sql_to_dataframe(sql_query, conn):
      import pandas as pd
      if not sql_query:
          return "Invalid SQL query"
      try:
          return pd.read_sql_query(sql_query, conn)
      except Exception as e:
          return "Error executing query"