Removed unnecessary error messages and improved windows compatibility

Fixed issue 741: (https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues/741)
The warnings are really annoying if you don't want to set system environment variables GOROOT/GOPATH.
You should anyway set the paths in the IDE.

Fixed issue 752: (https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues/752)
Improved clarity by not showing a warning if there is no bin folder in the specified GOPATH because app-engine does not require a bin folder.

Also fixed the automatic preselection of the correct GOROOT in Windows when configuring the Go-SDK.