Fix inconsistency between SendResult and other results (#60)

Almost all other results:

* `NewProfileExporterV3Result`
* `SerializeResult`
* `PushTagResult`

expose a `...Result_Tag` named `..._RESULT_ERR` and an `err` field
inside their definition, so it's weird to use `failure` just for
`SendResult`, which otherwise looks exactly like all the others.

Here's how `ffi.h` looks after this change:

```diff
 typedef enum ddprof_ffi_SendResult_Tag {
   DDPROF_FFI_SEND_RESULT_HTTP_RESPONSE,
-  DDPROF_FFI_SEND_RESULT_FAILURE,
+  DDPROF_FFI_SEND_RESULT_ERR,
 } ddprof_ffi_SendResult_Tag;

 typedef struct ddprof_ffi_SendResult {
@@ -150,7 +150,7 @@
       struct ddprof_ffi_HttpStatus http_response;
     };
     struct {
-      struct ddprof_ffi_Vec_u8 failure;
+      struct ddprof_ffi_Vec_u8 err;
     };
   };
 } ddprof_ffi_SendResult;
```

The `ParseTagsResult` is still inconsistent (uses `error_message`)
but that one is inconsistent in other ways (doesn't expose a tag,
uses a pointer to a Vec_u8), so I think it's not a bad idea that
`error_message` is different as it needs to be handled differently.

ddprof-ffi/src/exporter.rs

@@ -16,7 +16,7 @@ use std::str::FromStr;
 #[repr(C)]
 pub enum SendResult {
     HttpResponse(HttpStatus),
-    Failure(crate::Vec<u8>),
+    Err(crate::Vec<u8>),
 }
 
 #[repr(C)]
@@ -202,15 +202,15 @@ pub unsafe extern "C" fn profile_exporter_send(
     let exp_ptr = match exporter {
         None => {
             let buf: &[u8] = b"Failed to export: exporter was null";
-            return SendResult::Failure(crate::Vec::from(Vec::from(buf)));
+            return SendResult::Err(crate::Vec::from(Vec::from(buf)));
         }
         Some(e) => e,
     };
 
     let request_ptr = match request {
         None => {
             let buf: &[u8] = b"Failed to export: request was null";
-            return SendResult::Failure(crate::Vec::from(Vec::from(buf)));
+            return SendResult::Err(crate::Vec::from(Vec::from(buf)));
         }
         Some(req) => req,
     };
@@ -223,7 +223,7 @@ pub unsafe extern "C" fn profile_exporter_send(
         Ok(HttpStatus(response.status().as_u16()))
     }() {
         Ok(code) => SendResult::HttpResponse(code),
-        Err(err) => SendResult::Failure(err.into()),
+        Err(err) => SendResult::Err(err.into()),
     }
 }
 

examples/ffi/exporter.cpp

@@ -147,8 +147,8 @@ int main(int argc, char *argv[]) {
   int exit_code = 0;
   ddprof_ffi_SendResult send_result =
       ddprof_ffi_ProfileExporterV3_send(exporter, request, cancel);
-  if (send_result.tag == DDPROF_FFI_SEND_RESULT_FAILURE) {
-    print_error("Failed to send profile: ", send_result.failure);
+  if (send_result.tag == DDPROF_FFI_SEND_RESULT_ERR) {
+    print_error("Failed to send profile: ", send_result.err);
     exit_code = 1;
   } else {
     printf("Response code: %d\n", send_result.http_response.code);