Use standard form of error strings in AppendFileOperation.
Use standard form of error strings in AppendFileOperation.

Change-Id: I49a3e987b509cf833d92c732aa48b81ae260eb45
Reviewed-by: Karsten Heimrich <81d67632fe4bf9afb88a400b41aa25e19a1d2a12@digia.com>

olear/qtifw,olear/qtifw,olear/qtifw,olear/qtifw