Add @CanIgnoreReturnValue to MockInputStreamHelper.injectExceptionAt(int) (#4275)

Author: kluever

firebase-storage/src/testUtil/java/com/google/firebase/storage/network/MockInputStreamHelper.java

@@ -14,6 +14,7 @@
 
 package com.google.firebase.storage.network;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -37,6 +38,7 @@ public MockInputStreamHelper(final byte[] responseData) {
     this.injectExceptions = new TreeSet<>();
   }
 
+  @CanIgnoreReturnValue
   public MockInputStreamHelper injectExceptionAt(int bytePos) {
     if (opened) {
       throw new IllegalStateException("Can't add exception points after reading from stream.");