Add Java DSL Flow#mapError version without PartialFunction (#24992) (#26310)

* Add an overloaded version of the Flow#mapError (Java DSL) which does not use a Scala PartialFunction.
* Add test verifying mapError matching on parent class
* Add to Source, SubSource and SubFlow as well

akka-stream/src/main/scala/akka/stream/javadsl/Source.scala

@@ -1847,6 +1847,30 @@ final class Source[Out, Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[
   def mapError(pf: PartialFunction[Throwable, Throwable]): javadsl.Source[Out, Mat] =
     new Source(delegate.mapError(pf))
 
+  /**
+   * While similar to [[recover]] this operator can be used to transform an error signal to a different one *without* logging
+   * it as an error in the process. So in that sense it is NOT exactly equivalent to `recover(t => throw t2)` since recover
+   * would log the `t2` error.
+   *
+   * Since the underlying failure signal onError arrives out-of-band, it might jump over existing elements.
+   * This operator can recover the failure signal, but not the skipped elements, which will be dropped.
+   *
+   * Similarly to [[recover]] throwing an exception inside `mapError` _will_ be logged.
+   *
+   * '''Emits when''' element is available from the upstream or upstream is failed and pf returns an element
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes or upstream failed with exception pf can handle
+   *
+   * '''Cancels when''' downstream cancels
+   *
+   */
+  def mapError[E <: Throwable](clazz: Class[E], f: function.Function[E, Throwable]): javadsl.Source[Out, Mat] =
+    mapError {
+      case err if clazz.isInstance(err) => f(clazz.cast(err))
+    }
+
   /**
    * RecoverWith allows to switch to alternative Source on flow failure. It will stay in effect after
    * a failure has been recovered so that each time there is a failure it is fed into the `pf` and a new