Propagate MediaSource loading errors to the player.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127551577

library/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -15,6 +15,8 @@
  */
 package com.google.android.exoplayer2.source;
 
+import java.io.IOException;
+
 /**
  * Concatenates multiple {@link MediaSource}s.
  */
@@ -50,7 +52,7 @@ public final class ConcatenatingMediaSource implements MediaSource {
   }
 
   @Override
-  public MediaPeriod createPeriod(int index) {
+  public MediaPeriod createPeriod(int index) throws IOException {
     int sourceCount = 0;
     for (MediaSource mediaSource : mediaSources) {
       int count = mediaSource.getPeriodCount();

library/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -15,6 +15,8 @@
  */
 package com.google.android.exoplayer2.source;
 
+import java.io.IOException;
+
 /**
  * A source of media consisting of one or more {@link MediaPeriod}s.
  */
@@ -30,8 +32,6 @@ public interface MediaSource {
    */
   void prepareSource();
 
-  // TODO add void maybeThrowError() throws IOException;
-
   /**
    * Returns the number of periods in the source, or {@link #UNKNOWN_PERIOD_COUNT} if the number
    * of periods is not yet known.
@@ -46,8 +46,10 @@ public interface MediaSource {
    *     period count is {@link #UNKNOWN_PERIOD_COUNT}.
    * @return A {@link MediaPeriod}, or {@code null} if the source at the specified index is not yet
    *     available.
+   * @throws IOException If there is an error that's preventing the source from becoming prepared or
+   *     creating periods.
    */
-  MediaPeriod createPeriod(int index);
+  MediaPeriod createPeriod(int index) throws IOException;
 
   /**
    * Releases the source.

library/src/main/java/com/google/android/exoplayer2/source/MergingMediaSource.java

@@ -17,6 +17,8 @@ package com.google.android.exoplayer2.source;
 
 import com.google.android.exoplayer2.util.Assertions;
 
+import java.io.IOException;
+
 /**
  * Merges multiple {@link MediaPeriod} instances.
  * <p>
@@ -55,7 +57,7 @@ public final class MergingMediaSource implements MediaSource {
   }
 
   @Override
-  public MediaPeriod createPeriod(int index) {
+  public MediaPeriod createPeriod(int index) throws IOException {
     MediaPeriod[] periods = new MediaPeriod[mediaSources.length];
     for (int i = 0; i < periods.length; i++) {
       periods[i] = mediaSources[i].createPeriod(index);

library/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -113,8 +113,9 @@ public final class DashMediaSource implements MediaSource {
   }
 
   @Override
-  public MediaPeriod createPeriod(int index) {
+  public MediaPeriod createPeriod(int index) throws IOException {
     if (periods == null) {
+      loader.maybeThrowError();
       return null;
     }
     return periods[index];