Remove exclamation marks from messages

Exclamation marks should usually be avoided in message strings (even in
error messages).
See also: http://qt-project.org/wiki/UI_Text_Conventions and other
HIG guidelines.

Change-Id: I0a4cc7c03cf64b130a2b32c1d77143e80e3e4024
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>