Exception raised in read-only mode when reading some cells
*Created originally on Bitbucket by [CharlieC](https://bitbucket.org/%7B32d660d5-f934-4ce5-a7b0-52750378fae1%7D/)*
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>
```
issue