Strong typed API for synchronous TDLib method execution in Java inter…

…face

`Client.execute` in Java interface is now strongly typed: returned TDLib object type depends on the return type defined in the corresponding Function class.

When TDLib error is occurred, method now throws `Client.ExecutionError`.

This change adds compile-time protection against return type change and allows using this pattern:

```
try {
  TdApi.SpecificReturnType result = Client.execute(function);
  // work with strongly typed resultl without casting and type checks
} catch (Client.ExecutionError error) {
  // Handle error
}
```

example/java/org/drinkless/tdlib/Client.java

@@ -62,6 +62,24 @@ public interface LogMessageHandler {
         void onLogMessage(int verbosityLevel, String message);
     }
 
+    /**
+     * Exception class thrown when TDLib error occurred while performing {@link #execute(TdApi.Function)}.
+     */
+    public static class ExecutionError extends Throwable {
+        /**
+         * Original TDLib error occurred when performing one of the synchronous functions.
+         */
+        public final TdApi.Error error;
+
+        /**
+         * @param error TDLib error occurred while performing {@link #execute(TdApi.Function)}.
+         */
+        ExecutionError (TdApi.Error error) {
+            super(error.code + ": " + error.message);
+            this.error = error;
+        }
+    }
+
     /**
      * Sends a request to the TDLib.
      *
@@ -102,8 +120,17 @@ public void send(TdApi.Function query, ResultHandler resultHandler) {
      * @return request result.
      * @throws NullPointerException if query is null.
      */
-    public static TdApi.Object execute(TdApi.Function query) {
-        return nativeClientExecute(query);
+    public static <T extends TdApi.Object> T execute(TdApi.Function<T> query) throws ExecutionError {
+        TdApi.Object object = nativeClientExecute(query);
+        if (object instanceof TdApi.Error) {
+            throw new ExecutionError((TdApi.Error) object);
+        }
+        if (object == null) {
+            // unreachable
+            throw new NullPointerException();
+        }
+        //noinspection unchecked
+        return (T) object;
     }
 
     /**