title: File Exists and Exception Handling in U-Sql
body: <p>Two questions</p>
  
  <ul>
  <li><p>How to check file exists or not before EXTRACT?
  we have scenario where new inputs file is generated every day for catalog data. we need to merge new input with d-1 file. before merge we what to make sure that new input file exists at source location</p></li>
  <li><p>does u-sql supports try...catch block?</p></li>
  </ul>