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/jsonrpc.h

@@ -4,6 +4,7 @@
 #include <bitcoin/chainparams.h>
 #include <ccan/autodata/autodata.h>
 #include <ccan/list/list.h>
+#include <common/errcode.h>
 #include <common/json.h>
 #include <lightningd/json_stream.h>
 #include <stdarg.h>
@@ -96,7 +97,7 @@ struct json_stream *json_stream_success(struct command *cmd);
  * You need to json_object_end() once you're done!
  */
 struct json_stream *json_stream_fail(struct command *cmd,
-				     int code,
+				     errcode_t code,
 				     const char *errmsg);
 
 /**
@@ -108,7 +109,7 @@ struct json_stream *json_stream_fail(struct command *cmd,
  * This is used by command_fail(), which doesn't add any JSON data.
  */
 struct json_stream *json_stream_fail_nodata(struct command *cmd,
-					    int code,
+					    errcode_t code,
 					    const char *errmsg);
 
 /* These returned values are never NULL. */