Summary: Add context when LLMModule load fails to aid in debugging (#16174)

Test Plan:

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### Summary
[PLEASE REMOVE] See [CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

[PLEASE REMOVE] If this PR introduces a fix or feature that should be
the upcoming release notes, please add a "Release notes: <area>" label.
For a list of available release notes labels, check out
[CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Co-authored-by: Github Executorch <github_executorch@arm.com>

Author: psiddh

extension/android/jni/jni_layer_llama.cpp

@@ -23,6 +23,8 @@
 #include <executorch/runtime/platform/platform.h>
 #include <executorch/runtime/platform/runtime.h>
 
+#include <executorch/extension/android/jni/jni_helper.h>
+
 #if defined(ET_USE_THREADPOOL)
 #include <executorch/extension/threadpool/cpuinfo_utils.h>
 #include <executorch/extension/threadpool/threadpool.h>
@@ -404,12 +406,29 @@ class ExecuTorchLlmJni : public facebook::jni::HybridClass<ExecuTorchLlmJni> {
   }
 
   jint load() {
+    int result = -1;
+    std::stringstream ss;
+
     if (model_type_category_ == MODEL_TYPE_CATEGORY_MULTIMODAL) {
-      return static_cast<jint>(multi_modal_runner_->load());
+      result = static_cast<jint>(multi_modal_runner_->load());
+      if (result != 0) {
+        ss << "Failed to load multimodal runner: [" << result << "]";
+      }
     } else if (model_type_category_ == MODEL_TYPE_CATEGORY_LLM) {
-      return static_cast<jint>(runner_->load());
+      result = static_cast<jint>(runner_->load());
+      if (result != 0) {
+        ss << "Failed to load llm runner: [" << result << "]";
+      }
+    } else {
+      ss << "Invalid model type category: " << model_type_category_
+         << ". Valid values are: " << MODEL_TYPE_CATEGORY_LLM << " or "
+         << MODEL_TYPE_CATEGORY_MULTIMODAL;
+    }
+    if (result != 0) {
+      executorch::jni_helper::throwExecutorchException(
+          static_cast<uint32_t>(Error::InvalidArgument), ss.str().c_str());
     }
-    return static_cast<jint>(Error::InvalidArgument);
+    return result; // 0 on success to keep backward compatibility
   }
 
   static void registerNatives() {