bind: error for packages with no exported names.

For error message improvement mentioned in golang/go#12842

Change-Id: I066ae9dc9415b34bf93128098199bbb935b7dedc
Reviewed-on: https://go-review.googlesource.com/15410
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/genjava.go

@@ -668,11 +668,13 @@ func (g *javaGen) gen() error {
 
 	scope := g.pkg.Scope()
 	names := scope.Names()
+	hasExported := false
 	for _, name := range names {
 		obj := scope.Lookup(name)
 		if !obj.Exported() {
 			continue
 		}
+		hasExported = true
 
 		switch o := obj.(type) {
 		// TODO(crawshaw): case *types.Var:
@@ -700,6 +702,9 @@ func (g *javaGen) gen() error {
 			g.errorf("unsupported exported type: %T", obj)
 		}
 	}
+	if !hasExported {
+		g.errorf("no exported names in the package %q", g.pkg.Path())
+	}
 
 	for i, name := range funcs {
 		g.Printf("private static final int CALL_%s = %d;\n", name, i+1)