title: Lookup with Missing Labels
body: <p>I have a code that uses a dataframe to look up a value (P) given it's column label (X):</p>
  
  <pre><code>df_1 = pd.DataFrame({'X': [1,2,3,1,1,2,1,3,2,1]})
  
  df_2 = pd.DataFrame({ 1 : [1,2,3,4,1,2,3,4,1,2],
                        2 : [4,1,2,3,4,1,2,1,2,3],
                        3 : [2,3,4,1,2,3,4,1,2,5]})
  
  df_1['P'] = df_2 .lookup(df_1.index, df_1['X'])
  </code></pre>
  
  <p>When I give it a label in df_1 but don't include that label in df_2, like this:</p>
  
  <pre><code>df_1 = pd.DataFrame({'X': [7,2,3,1,1,2,1,3,2,1]})
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>KeyError: 'One or more column labels was not found'
  </code></pre>
  
  <p>How can I skip those ones please, to get:</p>
  
  <pre><code>   X  P
  0  7  NaN
  1  2  1
  2  3  4
  3  1  4
  4  1  1
  5  2  1
  6  1  3
  7  3  1
  8  2  2
  9  1  2
  </code></pre>