handle exceptions when comparing dataframes with single value

[rishsriv]

Previously, the question What's the average duration between departure and arrival times minus 34 minutes? Convert from UNIX to regular datetime., we always got a 'bool' object has no attribute 'all' error

Using the traceback, it seems like the error was coming from this section of the code:

Traceback (most recent call last):
  File "/Users/rishabh/defog/sql-eval/eval/api_runner.py", line 54, in process_row
    exact_match, correct = compare_query_results(
  File "/Users/rishabh/defog/sql-eval/eval/eval.py", line 348, in compare_query_results
    if compare_df(
  File "/Users/rishabh/defog/sql-eval/eval/eval.py", line 250, in compare_df
    if df_gold.shape == df_gen.shape and (df_gold.values == df_gen.values).all():
AttributeError: 'bool' object has no attribute 'all'

This fixes this with better exception handling