cmd/cover: don't ignore os.Create error

Failing to create the output file would give confusing errors such as:

	cover: invalid argument

Also do out.Close() even if Execute() errored.

Fixes #17951.

Change-Id: I897e1d31f7996871c54fde7cb09614cafbf6c3fc
Reviewed-on: https://go-review.googlesource.com/33278Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/cmd/cover/html.go

@@ -64,9 +64,12 @@ func htmlOutput(profile, outfile string) error {
 	} else {
 		out, err = os.Create(outfile)
 	}
+	if err != nil {
+		return err
+	}
 	err = htmlTemplate.Execute(out, d)
-	if err == nil {
-		err = out.Close()
+	if err2 := out.Close(); err == nil {
+		err = err2
 	}
 	if err != nil {
 		return err