title: Find data that are not letting me convert string to float
body: <p>I am trying to convert a column of Object dtype to float64</p>
  
  <p>Pandas version - 2.21</p>
  
  <p>I tried using convert_objects() to force rows that cannot convert to NaN and was successful at converting my column to a float64</p>
  
  <p>I want to know what rows/data did not allow me to convert it into a float64.
  Is there a function out their that can do that ?</p>
  
  <p>For example :</p>
  
  <pre><code>col1
  
  2015
  2016
  NaN
  NaN
  3005
  i_am_a_string
  4006
  another_string
  5008
  4005
  
  df['col1'].astype(float64) 
  
  FAILED!! because the column has string data and cannot convert them all to float64
  
  </code></pre>
  
  <p>My desired output I want to see those strings</p>
  
  <pre><code>i_am_a_string
  another_string
  </code></pre>