Cleanup: move error reporting function declarations to errorhelper.h

Move the declarations of the "report_error()" and "set_error_cb()"
functions and the "verbose" variable to errorhelper.h.
Thus, error-reporting translation units don't have to import the
big dive.h header file.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

mobile-widgets/qmlmanager.cpp

@@ -26,6 +26,7 @@
 #include "qt-models/messagehandlermodel.h"
 #include "qt-models/tankinfomodel.h"
 #include "core/device.h"
+#include "core/errorhelper.h"
 #include "core/file.h"
 #include "core/qthelper.h"
 #include "core/qt-gui.h"