Avoid panic if printer is off or unreachable for some reason.

Closes #16
Closes #17
Closes #15

packages/ipp/messages.go

@@ -115,6 +115,7 @@ const (
 	SuccessfulOkIgnoredOrSubstitutedAttributes statusCode = 0x0001
 	SuccessfulOkConflictingAttributes          statusCode = 0x0002
 	ClientErrorBadRequest                      statusCode = 0x0400
+	ServerErrorServiceUnavailable              statusCode = 0x0502
 )
 
 type versionNumber uint16

packages/ipp/request.go

@@ -65,6 +65,7 @@ func (r *Request) RequestID() uint32 {
 	return r.header.requestID
 }
 
+// Operation returns the operation is of the request.
 func (r *Request) Operation() OperationID {
 	return r.header.operationID
 }

packages/ipp/response.go

@@ -52,6 +52,10 @@ func (r Response) String() string {
 	return r.header.String() + "\n" + r.a.String()
 }
 
+func (r Response) Header() ippResponseHeader {
+	return r.header
+}
+
 // Marshal converts the response object to a wire formatted byte stream.
 func (r *Response) Marshal() []byte {
 	a := make([]byte, 0, 20)