Show build details when a build fails

Closes #4

builder.go

@@ -56,7 +56,7 @@ func (b *builder) Build() error {
 
 	output, err := command.CombinedOutput()
 	if err != nil {
-		return err
+		return fmt.Errorf("build failed with %v\n%s", err, output)
 	}
 
 	if !command.ProcessState.Success() {

builder_test.go

@@ -38,10 +38,15 @@ func TestBuilderFailureBuild(t *testing.T) {
 		t.Fatalf("couldn't get current working directory: %v", err)
 	}
 
+	absPathBuild := filepath.Join(wd, dir)
+
 	b := NewBuilder(dir, bin, wd, bArgs)
 	err = b.Build()
 	assert.NotNil(t, err, "build error")
-	assert.Equal(t, err.Error(), "exit status 2")
+	assert.Equal(t, err.Error(), "build failed with exit status 2\n"+
+		"# _"+absPathBuild+"\n"+
+		"./main.go:4:6: func main must have no arguments and no return values\n"+
+		"./main.go:5:1: missing return at end of function\n")
 }
 
 func TestBuilderDefaultBinName(t *testing.T) {