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/errorhelper.h

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef ERROR_HELPER_H
+#define ERROR_HELPER_H
+
+// error reporting functions
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern int verbose;
+extern int report_error(const char *fmt, ...);
+extern void set_error_cb(void(*cb)(char *));	// Callback takes ownership of passed string
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif