Fix exception message

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190215051

library/core/src/main/java/com/google/android/exoplayer2/source/MergingMediaPeriod.java

@@ -104,7 +104,7 @@ import java.util.IdentityHashMap;
       if (i == 0) {
       if (i == 0) {
         positionUs = selectPositionUs;
         positionUs = selectPositionUs;
       } else if (selectPositionUs != positionUs) {
       } else if (selectPositionUs != positionUs) {
-        throw new IllegalStateException("Children enabled at different positions");
+        throw new IllegalStateException("Children enabled at different positions.");
       }
       }
       boolean periodEnabled = false;
       boolean periodEnabled = false;
       for (int j = 0; j < selections.length; j++) {
       for (int j = 0; j < selections.length; j++) {
@@ -161,7 +161,7 @@ import java.util.IdentityHashMap;
     // Periods other than the first one are not allowed to report discontinuities.
     // Periods other than the first one are not allowed to report discontinuities.
     for (int i = 1; i < periods.length; i++) {
     for (int i = 1; i < periods.length; i++) {
       if (periods[i].readDiscontinuity() != C.TIME_UNSET) {
       if (periods[i].readDiscontinuity() != C.TIME_UNSET) {
-        throw new IllegalStateException("Child reported discontinuity");
+        throw new IllegalStateException("Child reported discontinuity.");
       }
       }
     }
     }
     // It must be possible to seek enabled periods to the new position, if there is one.
     // It must be possible to seek enabled periods to the new position, if there is one.
@@ -169,7 +169,7 @@ import java.util.IdentityHashMap;
       for (MediaPeriod enabledPeriod : enabledPeriods) {
       for (MediaPeriod enabledPeriod : enabledPeriods) {
         if (enabledPeriod != periods[0]
         if (enabledPeriod != periods[0]
             && enabledPeriod.seekToUs(positionUs) != positionUs) {
             && enabledPeriod.seekToUs(positionUs) != positionUs) {
-          throw new IllegalStateException("Children seeked to different positions");
+          throw new IllegalStateException("Unexpected child seekToUs result.");
         }
         }
       }
       }
     }
     }
@@ -187,7 +187,7 @@ import java.util.IdentityHashMap;
     // Additional periods must seek to the same position.
     // Additional periods must seek to the same position.
     for (int i = 1; i < enabledPeriods.length; i++) {
     for (int i = 1; i < enabledPeriods.length; i++) {
       if (enabledPeriods[i].seekToUs(positionUs) != positionUs) {
       if (enabledPeriods[i].seekToUs(positionUs) != positionUs) {
-        throw new IllegalStateException("Children seeked to different positions");
+        throw new IllegalStateException("Unexpected child seekToUs result.");
       }
       }
     }
     }
     return positionUs;
     return positionUs;