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

wire/fromwire.c

@@ -9,8 +9,10 @@
 #include <ccan/crypto/siphash24/siphash24.h>
 #include <ccan/endian/endian.h>
 #include <ccan/mem/mem.h>
+#include <ccan/short_types/short_types.h>
 #include <ccan/tal/str/str.h>
 #include <common/amount.h>
+#include <common/errcode.h>
 #include <common/node_id.h>
 #include <common/type_to_string.h>
 #include <common/utils.h>
@@ -165,12 +167,12 @@ bool fromwire_bool(const u8 **cursor, size_t *max)
 	return ret;
 }
 
-int fromwire_int(const u8 **cursor, size_t *max)
+errcode_t fromwire_errcode_t(const u8 **cursor, size_t *max)
 {
-	int ret;
+	errcode_t ret;
+
+	ret = (s32)fromwire_u32(cursor, max);
 
-	if (!fromwire(cursor, max, &ret, sizeof(ret)))
-		return 0;
 	return ret;
 }