Add support for error values

[Shimuuar]

Error values are not decode currently. They have type tag "e" and string value. Following errors do occur in excel: #NULL!, #DIV/0, #VALUE!, #REF!, #NAME?, #NUM!, #N/A. I'm not sure whether this list is exahaustive

Example cells with errors:

      <c r="A1" t="e">
        <v>#VALUE!</v>
      </c>

      <c r="A3" t="e">
        <f>0/0</f>
        <v>#DIV/0!</v>
      </c>