title: What causes "UserWarning: Discarded range with reserved name" - openpyxl
body: <p>I have a simple EXCEL-sheet with names of cities in column A and I want to extract them and put them in a list:</p>
  
  <pre><code>def getCityfromEXCEL():
      wb = load_workbook(filename='test.xlsx', read_only=True)
      ws = wb['Sheet1']
      cityList = []
  
      for i in range(2, ws.get_highest_row()+1):
          acell = "A"+str(i)
          cityString = ws[acell].value
          city = ftfy.fix_text_encoding(cityString)            
          cityList.append(city)
  
  getCityfromEXCEL()
  </code></pre>
  
  <p>With a small file that worked perfectly (70 rows). Now I'm processing a big file (8300 rows) and it gives me this error:</p>
  
  <pre><code>/Library/Python/2.7/site-packages/openpyxl/workbook/names/named_range.py:121: UserWarning: Discarded range with reserved name
    warnings.warn("Discarded range with reserved name")
  </code></pre>
  
  <p>but it does not abort. It just does not seem to continue anymore. Can someone tell me what might cause the error? Is it something in the .xlsx? Any special hints what I can look for?</p>