x/tools/cmd/goimports: show detailed error when directory cannot be opened

[ysmolski]

When goimports cannot some directory it does not tell me which directory is to blaim. It can lead to cumbersome debugging.

Instead of

% goimports -e main.go
2018/02/27 17:45:40 goimports: scanning directory /h/t/go/src: permission denied

I want to see

% goimports -e main.go
2018/02/27 17:45:40 goimports: scanning directory /h/t/go/src: permission denied while opening dir: /h/t/go/src/dir/denied

Refers to #16890 which is supposed to be full fix of the situation (do not exit from goimports in dir open error)

[gopherbot]

Change https://golang.org/cl/97359 mentions this issue: x/tools/cmd/goimports: show detailed error when directory cannot be opened