godoc: set the GOROOT path properly in cmdline mode

- Setting the GOROOT build path to the value passed from the command line.

- Clarified the return values to named parameters for extra clarity.

- And while here, added some missed out error handling. Just logging the error
to preserve original behavior.

Fixes golang/go#13296

Change-Id: I91427eee790928a3cfb51ae207747e9a17bd5496
Reviewed-on: https://go-review.googlesource.com/110275
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

cmd/godoc/main.go

@@ -356,6 +356,7 @@ func main() {
 		return
 	}
 
+	build.Default.GOROOT = *goroot
 	if err := godoc.CommandLine(os.Stdout, fs, pres, flag.Args()); err != nil {
 		log.Print(err)
 	}

godoc/cmdline.go

@@ -134,18 +134,25 @@ func CommandLine(w io.Writer, fs vfs.NameSpace, pres *Presentation, args []strin
 // for this.  That is, if we get passed a directory like the above, we map that
 // directory so that getPageInfo sees it as /target.
 // Returns the absolute and relative paths.
-func paths(fs vfs.NameSpace, pres *Presentation, path string) (string, string) {
+func paths(fs vfs.NameSpace, pres *Presentation, path string) (abspath, relpath string) {
 	if filepath.IsAbs(path) {
 		fs.Bind(target, vfs.OS(path), "/", vfs.BindReplace)
 		return target, target
 	}
 	if build.IsLocalImport(path) {
-		cwd, _ := os.Getwd() // ignore errors
+		cwd, err := os.Getwd()
+		if err != nil {
+			log.Printf("error while getting working directory: %v", err)
+		}
 		path = filepath.Join(cwd, path)
 		fs.Bind(target, vfs.OS(path), "/", vfs.BindReplace)
 		return target, target
 	}
-	if bp, _ := build.Import(path, "", build.FindOnly); bp.Dir != "" && bp.ImportPath != "" {
+	bp, err := build.Import(path, "", build.FindOnly)
+	if err != nil {
+		log.Printf("error while importing build package: %v", err)
+	}
+	if bp.Dir != "" && bp.ImportPath != "" {
 		fs.Bind(target, vfs.OS(bp.Dir), "/", vfs.BindReplace)
 		return target, bp.ImportPath
 	}