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>

core/qt-init.cpp

@@ -5,7 +5,7 @@
 #include <QLibraryInfo>
 #include <QTextCodec>
 #include "qthelper.h"
-#include "dive.h" // for "verbose"
+#include "errorhelper.h"
 #include "core/settings/qPref.h"
 
 char *settings_suffix = NULL;