input/async: add attribute "postponed_exception"

Will replace "postponed_error".

src/input/AsyncInputStream.cxx

@@ -88,6 +88,12 @@ AsyncInputStream::Resume()
 bool
 AsyncInputStream::Check(Error &error)
 {
+	if (postponed_exception) {
+		auto e = std::move(postponed_exception);
+		postponed_exception = std::exception_ptr();
+		std::rethrow_exception(e);
+	}
+
 	bool success = !postponed_error.IsDefined();
 	if (!success) {
 		error = std::move(postponed_error);
@@ -181,6 +187,7 @@ bool
 AsyncInputStream::IsAvailable()
 {
 	return postponed_error.IsDefined() ||
+		postponed_exception ||
 		IsEOF() ||
 		!buffer.IsEmpty();
 }

src/input/AsyncInputStream.hxx

@@ -26,6 +26,8 @@
 #include "util/CircularBuffer.hxx"
 #include "util/Error.hxx"
 
+#include <exception>
+
 /**
  * Helper class for moving asynchronous (non-blocking) InputStream
  * implementations to the I/O thread.  Data is being read into a ring
@@ -67,6 +69,8 @@ class AsyncInputStream : public InputStream {
 protected:
 	Error postponed_error;
 
+	std::exception_ptr postponed_exception;
+
 public:
 	/**
 	 * @param _buffer a buffer allocated with HugeAllocate(); the