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

lightningd/connect_control.c

@@ -2,6 +2,7 @@
 #include <ccan/fdpass/fdpass.h>
 #include <ccan/list/list.h>
 #include <ccan/tal/str/str.h>
+#include <common/errcode.h>
 #include <common/features.h>
 #include <common/json_command.h>
 #include <common/json_helpers.h>
@@ -231,7 +232,7 @@ void delay_then_reconnect(struct channel *channel, u32 seconds_delay,
 static void connect_failed(struct lightningd *ld, const u8 *msg)
 {
 	struct node_id id;
-	int errcode;
+	errcode_t errcode;
 	char *errmsg;
 	struct connect *c;
 	u32 seconds_to_delay;