go.tools/playground: return a better message for code starting with a shebang on non local mode
go.tools/playground: return a better message for code starting with a shebang on non local mode

LGTM=adg, dan.kortschak
R=adg, dan.kortschak, campoy
CC=golang-codereviews
https://codereview.appspot.com/119160043

webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools