go.tools/go/types: print meaningful panic for unknown expressions
go.tools/go/types: print meaningful panic for unknown expressions

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/81310045

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