curl/Handler: disallow OnData() to throw

This eliminates some complexity from class CurlRequest.

test/RunCurl.cxx

@@ -50,8 +50,12 @@ public:
 	}
 
 	void OnData(ConstBuffer<void> data) override {
-		if (fwrite(data.data, data.size, 1, stdout) != 1)
-			throw std::runtime_error("Failed to write");
+		try {
+			if (fwrite(data.data, data.size, 1, stdout) != 1)
+				throw std::runtime_error("Failed to write");
+		} catch (...) {
+			OnError(std::current_exception());
+		}
 	}
 
 	void OnEnd() override {