Implement QPromise::tapFail(handler)

docs/SUMMARY.md

@@ -11,6 +11,7 @@
   * [.isPending](qtpromise/qpromise/ispending.md)
   * [.isRejected](qtpromise/qpromise/isrejected.md)
   * [.tap](qtpromise/qpromise/tap.md)
+  * [.tapFail](qtpromise/qpromise/tapfail.md)
   * [.then](qtpromise/qpromise/then.md)
   * [.timeout](qtpromise/qpromise/timeout.md)
   * [.wait](qtpromise/qpromise/wait.md)

docs/qtpromise/api-reference.md

@@ -10,6 +10,7 @@
 * [`QPromise<T>::isPending`](qpromise/ispending.md)
 * [`QPromise<T>::isRejected`](qpromise/isrejected.md)
 * [`QPromise<T>::tap`](qpromise/tap.md)
+* [`QPromise<T>::tapFail`](qpromise/tapfail.md)
 * [`QPromise<T>::then`](qpromise/then.md)
 * [`QPromise<T>::timeout`](qpromise/timeout.md)
 * [`QPromise<T>::wait`](qpromise/wait.md)

docs/qtpromise/qpromise/tapfail.md

@@ -0,0 +1,21 @@
+## `QPromise<T>::tapFail`
+
+```
+QPromise<T>::tapFail(Function handler) -> QPromise<T>
+```
+
+This `handler` allows to observe errors of the `input` promise without handling them - similar to [`finally`](finally.md) but **only** called on rejections. The `output` promise has the same type as the `input` one but also the same value or error. However, if `handler` throws, `output` is rejected with the new exception.
+
+```cpp
+QPromise<int> input = {...}
+auto output = input.tapFail([](Error err) {
+    log(err);
+}).then([](int res) {
+    return process(res);
+}).fail([](Error err) {
+    handle(err);
+    return -1;
+});
+```
+
+If `handler` returns a promise (or QFuture), the `output` promise is delayed until the returned promise is resolved and under the same conditions: the delayed value is ignored, the error transmitted to the `output` promise.