do not show warnings if system env-vars GOROOT or GOPATH are not set.
Do not show error message if there is no bin folder in GOPATH.
Fixes confusion as in https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues/752
Fixed automatic detection of GOROOT when selecting SDK in Windows 7
Merge pull request #757 from hfinger/master

Removed unnecessary error messages and improved windows compatibility