Exception raised in read-only mode when reading some cells

Created originally on Bitbucket by CharlieC

openpyxl will raise an exception when trying to read a cell from a worksheet where there is no relevant row in the source file.

ie. ws['A60'] for the following source. ws.iter_rows() is fine.

#!xml

   <row r="59" spans="1:14" x14ac:dyDescent="0.25">
      <c r="A59" t="s">
        <v>99</v>
      </c>
    </row>
    <row r="61" spans="1:14" x14ac:dyDescent="0.25">
      <c r="A61" t="s">
        <v>94</v>
      </c>
    </row>