perf config: Check error cases of {show_spec, set}_config()

show_spec_config() and set_config() can be called multiple times
in the loop in cmd_config().

However, The error cases of them wasn't checked, so fix it.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1497671197-20450-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-config.c

@@ -225,10 +225,23 @@ int cmd_config(int argc, const char **argv)
 				break;
 			}
 
-			if (value == NULL)
+			if (value == NULL) {
 				ret = show_spec_config(set, var);
-			else
+				if (ret < 0) {
+					pr_err("%s is not configured: %s\n",
+					       var, config_filename);
+					free(arg);
+					break;
+				}
+			} else {
 				ret = set_config(set, config_filename, var, value);
+				if (ret < 0) {
+					pr_err("Failed to set '%s=%s' on %s\n",
+					       var, value, config_filename);
+					free(arg);
+					break;
+				}
+			}
 			free(arg);
 		}
 	}