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/notification.h

@@ -7,6 +7,7 @@
 #include <ccan/json_escape/json_escape.h>
 #include <ccan/time/time.h>
 #include <common/amount.h>
+#include <common/errcode.h>
 #include <common/node_id.h>
 #include <lightningd/htlc_end.h>
 #include <lightningd/jsonrpc.h>
@@ -62,7 +63,7 @@ void notify_sendpay_success(struct lightningd *ld,
 
 void notify_sendpay_failure(struct lightningd *ld,
 			    const struct wallet_payment *payment,
-			    int pay_errcode,
+			    errcode_t pay_errcode,
 			    const struct onionreply *onionreply,
 			    const struct routing_failure *fail,
 			    const char *errmsg);