Persistent settings reader: Emit proper warning on value read failure.

Fix annoying assertion when reading session files:

SOFT ASSERT: "simpleValue.isValid()" in file
.\persistentsettings.cpp, line 126 .

Change-Id: Ia73a7a4a6f7a28c20876daf11a527b638ee02d89
Reviewed-by: hjk <hjk121@nokiamail.com>