Add parameter to enable DataFramesNotEqualError to be raised without printing the differences between the two dataframes

[slavyolov]

When calling the assert_df_equality and assert_approx_df_equality it will be good to have the option to not display the get_string(). Sometimes the output might be to long or truncated.I think this could be a good addition.

Example of the idea:

DataFramesNotEqualError("\n" + t.get_string()) if full_log else DataFramesNotEqualError

Where full_log is boolean 

[MrPowers]

@slavyolov - thanks for opening the issue. I'm open to the suggestion, but not entirely sure I understand how the output will be different. Can you open a PR and show the output when full_log is true and the output when full_log is false, so I can better understand? Thanks!