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/error_accumulator.go

@@ -0,0 +1,44 @@
+package openai
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+type ErrorAccumulator interface {
+	Write(p []byte) error
+	Bytes() []byte
+}
+
+type errorBuffer interface {
+	io.Writer
+	Len() int
+	Bytes() []byte
+}
+
+type DefaultErrorAccumulator struct {
+	Buffer errorBuffer
+}
+
+func NewErrorAccumulator() ErrorAccumulator {
+	return &DefaultErrorAccumulator{
+		Buffer: &bytes.Buffer{},
+	}
+}
+
+func (e *DefaultErrorAccumulator) Write(p []byte) error {
+	_, err := e.Buffer.Write(p)
+	if err != nil {
+		return fmt.Errorf("error accumulator write error, %w", err)
+	}
+	return nil
+}
+
+func (e *DefaultErrorAccumulator) Bytes() (errBytes []byte) {
+	if e.Buffer.Len() == 0 {
+		return
+	}
+	errBytes = e.Buffer.Bytes()
+	return
+}