Raise an error when report is empty (#214)

* Raise an error when report is empty

* Add test :)

cmd/format-coverage.go

@@ -105,6 +105,10 @@ func (f CoverageFormatter) Save() error {
 		return errors.WithStack(err)
 	}
 
+	if len(rep.SourceFiles) == 0 {
+		return errors.WithStack(errors.New("could not find coverage info for source files"))
+	}
+
 	if f.writer == nil {
 		f.writer, err = writer(formatOptions.Output)
 		if err != nil {

cmd/format-coverage_test.go

@@ -0,0 +1,19 @@
+package cmd
+
+import (
+	"testing"
+
+	"github.com/codeclimate/test-reporter/formatters/clover"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_CoverageFormatter_Save_Fail_On_Empty_Report(t *testing.T) {
+	r := require.New(t)
+	a := CoverageFormatter{}
+	a.In = &clover.Formatter{}
+	a.In.Search("../formatters/clover/empty_example.xml")
+
+	err := a.Save()
+	r.Error(err)
+	r.Equal("could not find coverage info for source files", err.Error())
+}

formatters/clover/empty_example.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<coverage generated="1493754316">
+  <project timestamp="1493754316">
+    <metrics files="13" loc="937" ncloc="722" classes="12" methods="49" coveredmethods="37" conditionals="0" coveredconditionals="0" statements="306" coveredstatements="148" elements="355" coveredelements="185" />
+  </project>
+</coverage>