Use dedicated type for error codes

Before this patch we used `int` for error codes. The problem with
`int` is that we try to pass it to/from wire and the size of `int` is
not defined by the standard. So a sender with 4-byte `int` would write
4 bytes to the wire and a receiver with 2-byte `int` (for example) would
read just 2 bytes from the wire.

To resolve this:

* Introduce an error code type with a known size:
  `typedef s32 errcode_t`.

* Change all error code macros to constants of type `errcode_t`.
  Constants also play better with gdb - it would visualize the name of
  the constant instead of the numeric value.

* Change all functions that take error codes to take the new type
  `errcode_t` instead of `int`.

* Introduce towire / fromwire functions to send / receive the newly added
  type `errcode_t` and use it instead of `towire_int()`.

In addition:

* Remove the now unneeded `towire_int()`.

* Replace a hardcoded error code `-2` with a new constant
  `INVOICE_EXPIRED_DURING_WAIT` (903).

Changelog-Changed: The waitinvoice command would now return error code 903 to designate that the invoice expired during wait, instead of the previous -2

plugins/libplugin.h

@@ -4,6 +4,7 @@
 #include "config.h"
 
 #include <ccan/time/time.h>
+#include <common/errcode.h>
 #include <common/json.h>
 #include <common/json_command.h>
 #include <common/json_helpers.h>
@@ -74,7 +75,7 @@ void NORETURN plugin_err(const char *fmt, ...);
  * NULL, data can be NULL; otherwise it must be a JSON object. */
 struct command_result *WARN_UNUSED_RESULT
 command_done_err(struct command *cmd,
-		 int code,
+		 errcode_t code,
 		 const char *errmsg,
 		 const struct json_out *data);