CA-146660: prefix all errors messages with tap-err

This is consistent with the other path of logging (using sylog(3)), and
facilitates debugging.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
CA-146660: print error message instead of errno

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge branch 'CA-146660' into xs64bit

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #134 on xapi-project/blktap