[flang] Modify an IO format error message

F18 constraint C1308 is:

  For the G edit descriptor, e shall not be specified if w is zero.

For an edit descriptor such as 'G0.2E4', change the error message from:

  error: Unexpected 'e' in 'G0' edit descriptor

To:

  error: A 'G0' edit descriptor must not have an 'e' value