ggml-backend : add root cause in error message if loading backend library fails (#16172)

This PR adds additional information to an error message when loading backend library via ld_load_library() fails. This helps spotting why backend library did not load (missing library, missing dependency or unresolved symbol etc.).

ggml/src/ggml-backend-reg.cpp

@@ -135,6 +135,10 @@ static void * dl_get_sym(dl_handle * handle, const char * name) {
     return p;
 }
 
+static const char * dl_error() {
+    return "";
+}
+
 #else
 
 using dl_handle = void;
@@ -155,6 +159,11 @@ static void * dl_get_sym(dl_handle * handle, const char * name) {
     return dlsym(handle, name);
 }
 
+static const char * dl_error() {
+    const char *rslt = dlerror();
+    return rslt != nullptr ? rslt : "";
+}
+
 #endif
 
 using dl_handle_ptr = std::unique_ptr<dl_handle, dl_handle_deleter>;
@@ -240,7 +249,7 @@ struct ggml_backend_registry {
         dl_handle_ptr handle { dl_load_library(path) };
         if (!handle) {
             if (!silent) {
-                GGML_LOG_ERROR("%s: failed to load %s\n", __func__, path_str(path).c_str());
+                GGML_LOG_ERROR("%s: failed to load %s: %s\n", __func__, path_str(path).c_str(), dl_error());
             }
             return nullptr;
         }
@@ -530,7 +539,7 @@ static ggml_backend_reg_t ggml_backend_load_best(const char * name, bool silent,
                 if (filename.native().find(file_prefix) == 0 && ext == file_extension) {
                     dl_handle_ptr handle { dl_load_library(entry) };
                     if (!handle && !silent) {
-                        GGML_LOG_ERROR("%s: failed to load %s\n", __func__, path_str(entry.path()).c_str());
+                        GGML_LOG_ERROR("%s: failed to load %s: %s\n", __func__, path_str(entry.path()).c_str(), dl_error());
                     }
                     if (handle) {
                         auto score_fn = (ggml_backend_score_t) dl_get_sym(handle.get(), "ggml_backend_score");