title: Pandas to_numeric with option errors=ignore does not convert to float
body: <p>I wanted to convert pandas series from object to float but keep other strings as it is.</p>
  
  <p>Here is the code snippet :-</p>
  
  <pre><code>In [37]: df = pd.DataFrame(['-1.0', 'hello', '0.5'])
  
  In [38]: df[0]
  Out[38]: 
  0   -1.0
  1  hello
  2    0.5
  Name: 0, dtype: object
  </code></pre>
  
  <p>What I want is this :-</p>
  
  <pre><code>In [43]: df[0]
  Out[43]: 
  0     -1.0
  1    hello
  2      0.5
  Name: 0, dtype: float64
  </code></pre>
  
  <p>I tried to use this but it converts <code>hello</code> to <code>NaN</code> which I don't want :-</p>
  
  <pre><code>In [41]: pd.to_numeric(df[0], errors='coerce')
  Out[41]: 
  0   -1.0
  1    NaN
  2    0.5
  Name: 0, dtype: float64
  </code></pre>
  
  <p>I tried putting <code>errors='ignore'</code> but it doesn't convert object to float :-</p>
  
  <pre><code>In [44]: pd.to_numeric(df[0], errors='ignore')
  Out[44]: 
  0     -1.0
  1    hello
  2      0.5
  Name: 0, dtype: object
  </code></pre>
  
  <p>Could someone please help me out with this ?</p>
  
  <p>Thanks.</p>