be more specific when unloading a neb module

Print the current and expected api version number along with the error.
This gives a hint about wether the neb module is too new or too old.

src/naemon/nebmods.c

@@ -194,12 +194,14 @@ int neb_load_module(nebmodule *mod)
 	module_version_ptr = (int *)dlsym(mod->module_handle, "__neb_api_version");
 
 	/* check the module API version */
-	if (module_version_ptr == NULL || ((*module_version_ptr) != CURRENT_NEB_API_VERSION)) {
-
-		nm_log(NSLOG_RUNTIME_ERROR, "Error: Module '%s' is using an old or unspecified version of the event broker API.  Module will be unloaded.\n", mod->filename);
-
+	if (module_version_ptr == NULL) {
+		nm_log(NSLOG_RUNTIME_ERROR, "Error: Module '%s' did not specify a version of the event broker API. Module will be unloaded.\n", mod->filename);
+		neb_unload_module(mod, NEBMODULE_FORCE_UNLOAD, NEBMODULE_ERROR_API_VERSION);
+		return ERROR;
+	}
+	if ((*module_version_ptr) != CURRENT_NEB_API_VERSION) {
+		nm_log(NSLOG_RUNTIME_ERROR, "Error: Module '%s' is using an incompatible version (v%d) of the event broker API (current version v%d). Module will be unloaded.\n", mod->filename, *module_version_ptr, CURRENT_NEB_API_VERSION);
 		neb_unload_module(mod, NEBMODULE_FORCE_UNLOAD, NEBMODULE_ERROR_API_VERSION);
-
 		return ERROR;
 	}