Merge pull request #1 from visualfc/extpkgerr

gopmod: lookupExternPkg error

gopmod/module.go

@@ -17,6 +17,7 @@
 package gopmod
 
 import (
+	"fmt"
 	"log"
 	"path/filepath"
 	"runtime"
@@ -130,7 +131,7 @@ func (p *Module) lookupExternPkg(pkgPath string) (pkg *Package, err error) {
 			return
 		}
 	}
-	err = syscall.ENOENT
+	err = &MissingError{Path: pkgPath}
 	return
 }
 
@@ -210,4 +211,13 @@ func loadModFrom(mod module.Version, mode mod.Mode) (p *Module, err error) {
 	return Load(dir, mode)
 }
 
+type MissingError struct {
+	Path string
+}
+
+func (e *MissingError) Error() string {
+	return fmt.Sprintf(`no required module provides package %v; to add it:
+	gop get %v`, e.Path, e.Path)
+}
+
 // -----------------------------------------------------------------------------