go.tools/go/loader: make error message for massive failure (e.g. no "fmt") more concise.
go.tools/go/loader: make error message for massive failure (e.g. no "fmt") more concise.

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/126080043

webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools