[LinuxCNC/linuxcnc PR#152] interp_o_word.cc: remove unused `tmpFileName` variable

This fix came from another branch without the
ERS(NCEUNABLETOOPENFILE,tmpFileName); line; there, it probably
caused an unused variable compiler warning. Here, with that line, it
would do something else wacky.

Remove the variable and substitute the correct file name variable into
the error.