IO: add close cause property to byte channel

Author: Sergey Mashkov

core/kotlinx-coroutines-io/src/main/kotlin/kotlinx/coroutines/experimental/io/ByteBufferChannel.kt

@@ -93,6 +93,9 @@ internal class ByteBufferChannel(
     override var totalBytesWritten: Long = 0L
         private set
 
+    override val closedCause: Throwable?
+        get() = closed?.cause
+
     override fun close(cause: Throwable?): Boolean {
         if (closed != null) return false
         val newClosed = if (cause == null) ClosedElement.EmptyCause else ClosedElement(cause)

core/kotlinx-coroutines-io/src/main/kotlin/kotlinx/coroutines/experimental/io/ByteWriteChannel.kt

@@ -43,6 +43,11 @@ public interface ByteWriteChannel {
      */
     public val totalBytesWritten: Long
 
+    /**
+     * An closure cause exception or `null` if closed successfully or not yet closed
+     */
+    public val closedCause: Throwable?
+
     /**
      * Writes as much as possible and only suspends if buffer is full
      */