do not show error message if there is no src or bin folder in the specified GOPATH.
Fixes confusion as in https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues/752
issue https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues/741

remove warnings if GOROOT or GOPATH system environment wariables are not set.
* The plugin works perfectly without setting these system-wide env-variables.
* The GOROOT anyway has to be manually confirmed when configuring the Go-SDK.
* The GOPATH consists anyway of the project Workdir and can optionally be appended with a System-Wide GOPATH which can be modified in the plugin settings.