Fix error handling when the instrumented code does not compile

In these cases, it is expected that the JSON data is not available,
and there's no reason to exit prematurely. The proper error handling
ensures that the -keep option properly reports the location of the
temporary files, without relying on the -verbose option.

main.go

@@ -295,7 +295,13 @@ func (g *gobco) runGoTest() {
 }
 
 func (g *gobco) printOutput() {
-	conds := g.load(g.statsFilename)
+	conds, err := g.load(g.statsFilename)
+	if err != nil && g.exitCode != 0 {
+		return // skip silently
+	}
+	if err != nil {
+		g.logger.errf("%s", err)
+	}
 
 	cnt := 0
 	for _, c := range conds {
@@ -322,7 +328,7 @@ func (g *gobco) printOutput() {
 func (g *gobco) cleanUp() {
 	if g.keep {
 		g.errf("")
-		g.errf("the temporary files are in %s", g.tmpdir)
+		g.errf("gobco: the temporary files are in %s", g.tmpdir)
 	} else {
 		err := os.RemoveAll(g.tmpdir)
 		if err != nil {
@@ -331,9 +337,11 @@ func (g *gobco) cleanUp() {
 	}
 }
 
-func (g *gobco) load(filename string) []condition {
+func (g *gobco) load(filename string) ([]condition, error) {
 	file, err := os.Open(filename)
-	g.check(err)
+	if err != nil {
+		return nil, err
+	}
 
 	defer func() {
 		closeErr := file.Close()
@@ -345,7 +353,7 @@ func (g *gobco) load(filename string) []condition {
 	decoder.DisallowUnknownFields()
 	g.check(decoder.Decode(&data))
 
-	return data
+	return data, nil
 }
 
 func (g *gobco) printCond(cond condition) {
@@ -412,7 +420,7 @@ func (t goTest) run(
 
 	err := goTest.Run()
 	if err != nil {
-		e.errf("%s", err)
+		e.errf("go test %s: %s", arg.arg, err)
 		return 1
 	} else {
 		e.verbosef("Finished %s", cmdline)