run: Improve error diagnostics for chdir(2) failure.

In run mode, if the destination directory does not exist, dss prints
"No such file or directory" and exits, without telling the user (a)
it was a failed chdir(2) call that caused the error, and (b) the name
of the directory. This patch adds an error message containing this
information.

Since there is only one caller of dss_chdir(), let's get rid
of this public function in file.c and call chdir() directly from
change_to_dest_dir() of dss.c.