title: Laravel Excel handle error when file is open
body: <p>I'm using Laravel Excel 2.1.0 in a local project to write a row into an excel file.</p>
  
  <p>This is my code:</p>
  
  <pre><code>$filePath = storage_path('myfile.xls');
  $rows = \Excel::load($filePath, function($reader) {
      $sheet = $reader-&gt;sheet(0);
      $sheet-&gt;appendRow(
          array(
               'Hello'
          )
      );
  });
  </code></pre>
  
  <p>Everything works and a new line was appended to my file.</p>
  
  <p>Sometimes can happens the excel file is opened while user try to append a new line. In this case Laravel, rightly, show me this error:</p>
  
  <pre><code>fopen(mypath\myfile.xls): failed to open stream: Resource temporarily unavailable
  </code></pre>
  
  <p>How can I handle this error in order to skip the function and go on with my code without append the row?</p>