ggml : address review comments on alloc_buffer_n
- restore GGML_LOG_ERROR on buffer alloc / tensor init failure in the
default impl (name the failing tensor)
- check the malloc result and drop the _impl indirection in
ggml_backend_alloc_ctx_tensors_from_buft
- remove comments that restate the code
- fix the TAG_ALLOC_SHARED_BUFFER_SPLIT typo
Assisted-by: pi:llama.cpp/Qwen3.8-27B
