title: Stop pip from failing on single package when installing with requirements.txt
body: <p>I am installing packages from <code>requirements.txt</code></p>
  
  <pre><code>pip install -r requirements.txt
  </code></pre>
  
  <p>The <code>requirements.txt</code> file reads:</p>
  
  <pre><code>Pillow
  lxml
  cssselect
  jieba
  beautifulsoup
  nltk
  </code></pre>
  
  <p><code>lxml</code> is the only package failing to install and this leads to everything failing (expected results as pointed out by larsks in the comments). However, after <code>lxml</code> fails <code>pip</code> still runs through and downloads the rest of the packages. </p>
  
  <p>From what I understand the <code>pip install -r requirements.txt</code> command will fail if any of the packages listed in the <code>requirements.txt</code> fail to install.</p>
  
  <p>Is there any argument I can pass when running <code>pip install -r requirements.txt</code> to tell it to install what it can and skip the packages that it cannot, or to exit as soon as it sees something fail?</p>