build: Detect vendored js package and return error. (#572)
build: Detect vendored js package and return error. (#572)

This follows #538.

Vendoring github.com/gopherjs/gopherjs/js package is not supported, and can cause
hard to understand runtime errors. Better catch it earlier and print a clear error message.

Helps #569.
gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,albrow/gopherjs,albrow/gopherjs,gopherjs/gopherjs