Change error message in ObservableFromArray (ReactiveX#6461)

* Change error message in ObservableFromArray

Changed error message from "The $i th element is null" to "The element at index $i is null".

Solves ReactiveX#6460

* Change error messages in FlowableFromArray

Changed error messages from "array element is null" to "The element at index $i is null".

Solves ReactiveX#6460

Author: iruizm

src/main/java/io/reactivex/internal/operators/flowable/FlowableFromArray.java

@@ -126,7 +126,7 @@ void fastPath() {
                 }
                 T t = arr[i];
                 if (t == null) {
-                    a.onError(new NullPointerException("array element is null"));
+                    a.onError(new NullPointerException("The element at index " + i + " is null"));
                     return;
                 } else {
                     a.onNext(t);
@@ -156,7 +156,7 @@ void slowPath(long r) {
                     T t = arr[i];
 
                     if (t == null) {
-                        a.onError(new NullPointerException("array element is null"));
+                        a.onError(new NullPointerException("The element at index " + i + " is null"));
                         return;
                     } else {
                         a.onNext(t);
@@ -209,7 +209,7 @@ void fastPath() {
                 }
                 T t = arr[i];
                 if (t == null) {
-                    a.onError(new NullPointerException("array element is null"));
+                    a.onError(new NullPointerException("The element at index " + i + " is null"));
                     return;
                 } else {
                     a.tryOnNext(t);
@@ -239,7 +239,7 @@ void slowPath(long r) {
                     T t = arr[i];
 
                     if (t == null) {
-                        a.onError(new NullPointerException("array element is null"));
+                        a.onError(new NullPointerException("The element at index " + i + " is null"));
                         return;
                     } else {
                         if (a.tryOnNext(t)) {

src/main/java/io/reactivex/internal/operators/observable/ObservableFromArray.java

@@ -102,7 +102,7 @@ void run() {
             for (int i = 0; i < n && !isDisposed(); i++) {
                 T value = a[i];
                 if (value == null) {
-                    downstream.onError(new NullPointerException("The " + i + "th element is null"));
+                    downstream.onError(new NullPointerException("The element at index " + i + " is null"));
                     return;
                 }
                 downstream.onNext(value);