go.tools/go/types: fix nil-ptr deref in gcimporter
go.tools/go/types: fix nil-ptr deref in gcimporter

Also: Make error handling more explicit.

Possibly a fix for issue 5758.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/10520043

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