util: Standardize console output in the m5 writefile command.

util: Standardize console output in the m5 writefile command.

When the command reports an error, it should then exit(2) and not just
return as if everything worked. When printing the number of bytes
written or the file being opened, it should write this non-error message
to cout, and not cerr.

Also used proper capitalization and punctuation in a couple messages.

Change-Id: I2c0d6592357965ed2eee8f090c8b3d530b354b9f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27627
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>