move error_accumulator into internal pkg (#304) (#335)

* move error_accumulator into internal pkg (#304)

* move error_accumulator into internal pkg (#304)

* add a test for ErrTooManyEmptyStreamMessages in stream_reader (#304)

internal/test/failer.go

@@ -0,0 +1,21 @@
+package test
+
+import "errors"
+
+var (
+	ErrTestErrorAccumulatorWriteFailed = errors.New("test error accumulator failed")
+)
+
+type FailingErrorBuffer struct{}
+
+func (b *FailingErrorBuffer) Write(_ []byte) (n int, err error) {
+	return 0, ErrTestErrorAccumulatorWriteFailed
+}
+
+func (b *FailingErrorBuffer) Len() int {
+	return 0
+}
+
+func (b *FailingErrorBuffer) Bytes() []byte {
+	return []byte{}
+}