api/msgpack-rpc: Improve error infrastructure

- Add error type information to `Error`
- Rename `set_api_error` to `api_set_error` for consistency with other api_*
  functions/macros.
- Refactor the api_set_error macro to accept formatted strings and error types
- Improve error messages
- Wrap error messages with gettext macro
- Refactor msgpack-rpc serialization to transform Error instances into [type,
  message] arrays
- Add error type information to API metadata
- Normalize nvim->client and client->nvim error handling(change
  channel_send_call to accept an Error pointer instead of the `errored` boolean
  pointer)
- Use macro to initialize Error structures

src/nvim/api/private/helpers.h

@@ -8,10 +8,13 @@
 #include "nvim/memory.h"
 #include "nvim/lib/kvec.h"
 
-#define set_api_error(message, err)                \
-  do {                                             \
-    xstrlcpy(err->msg, message, sizeof(err->msg)); \
-    err->set = true;                               \
+#define api_set_error(err, errtype, ...)             \
+  do {                                               \
+    snprintf((err)->msg,                             \
+             sizeof((err)->msg),                     \
+             __VA_ARGS__);                           \
+    (err)->set = true;                               \
+    (err)->type = kErrorType##errtype;               \
   } while (0)
 
 #define OBJECT_OBJ(o) o