ipc,commands,config: Replace cmd_status enum with cmd_results struct.

In i3 the ipc reply will contain a human readable error message, and
this patch replicates that behaviour.

However, that error message is also useful for logging, which this
patch takes advantage of.

E.g. instead of logging errors directly in commands.c/checkargs, it is
fed back to the caller which eventually ends up logging everything with
maximum context available (config.c/read_config).

So instead of logging e.g. "Error on line 'exit'" it will now log:
"Error on line 'exit': Can't execute from config."

include/commands.h

@@ -1,22 +1,34 @@
 #ifndef _SWAY_COMMANDS_H
 #define _SWAY_COMMANDS_H
 #include <stdbool.h>
+#include <json-c/json.h>
 #include "config.h"
 
 
 enum  cmd_status {
 	CMD_SUCCESS,
-	CMD_FAILURE,
-	CMD_INVALID,
+	CMD_FAILURE, // was or at least could be executed
+	CMD_INVALID, // unknown or parse error
 	CMD_DEFER,
 	// Config Blocks
 	CMD_BLOCK_END,
 	CMD_BLOCK_MODE,
 };
 
-enum cmd_status handle_command(char *command);
+struct cmd_results {
+	enum cmd_status status;
+
+	const char *input;
+	char *error;
+};
+
+struct cmd_results *handle_command(char *command);
 // Handles commands during config
-enum cmd_status config_command(char *command);
+struct cmd_results *config_command(char *command);
+
+struct cmd_results *cmd_results_new(enum cmd_status status, const char* input, const char *error, ...);
+void free_cmd_results(struct cmd_results *results);
+const char *cmd_results_to_json(struct cmd_results *results);
 
 void remove_view_from_scratchpad();