Add an indication of whether the PNG/JPG image saving was successful or not,

see issue #528. Unfortunately the library used does not give any detailed
information about possible errors. In fact  stb_image_write.h (line 77)
tells:   "Each function returns 0 on failure and non-0 on success."

Author: akleine

examples/cli/main.cpp

@@ -1830,15 +1830,16 @@ int main(int argc, const char* argv[]) {
             if (results[i].data == nullptr) {
                 continue;
             }
+            int write_ok;
             std::string final_image_path = i > 0 ? base_path + "_" + std::to_string(i + 1) + file_ext : base_path + file_ext;
             if (is_jpg) {
-                stbi_write_jpg(final_image_path.c_str(), results[i].width, results[i].height, results[i].channel,
+                write_ok = stbi_write_jpg(final_image_path.c_str(), results[i].width, results[i].height, results[i].channel,
                                results[i].data, 90, get_image_params(params, params.seed + i).c_str());
-                printf("save result JPEG image to '%s'\n", final_image_path.c_str());
+                printf("save result JPEG image to '%s' (%s)\n", final_image_path.c_str(), write_ok==0 ? "failure":"success");
             } else {
-                stbi_write_png(final_image_path.c_str(), results[i].width, results[i].height, results[i].channel,
+                write_ok = stbi_write_png(final_image_path.c_str(), results[i].width, results[i].height, results[i].channel,
                                results[i].data, 0, get_image_params(params, params.seed + i).c_str());
-                printf("save result PNG image to '%s'\n", final_image_path.c_str());
+                printf("save result PNG image to '%s' (%s)\n", final_image_path.c_str(), write_ok==0 ? "failure":"success");
             }
         }
     }