[Bug-tar] Separate return code when file changes while being read

[Steve Bergman]

If a file changes while tar is reading it, tar returns the same code as
when there is a major problem like a tape write error.

This makes it unusable for automated nightly backups.  It is perfectly
reasonable to want to include, for example, log files in the backup,
which might be appended to while being read.  When this happens, it is
impossible for a script to tell if that is why it gets a nonzero code,
or if the tape drive is on fire.

Certainly, it is reasonable to return a nonzero code in the case that a
file changes while being read, but it should not be the *same* nonzero
code as for more serious problems.