Retry calls to Last.FM without MBIDs when if returns artist invalid (#1138)

* Call Last.FM's getInfo again without mbid when artist is not found

* Call Last.FM's getSimilar again without mbid when artist is not found

* Call Last.FM's getTopTracks again without mbid when artist is not found

tests/fake_http_client.go

@@ -0,0 +1,19 @@
+package tests
+
+import "net/http"
+
+type FakeHttpClient struct {
+	Res          http.Response
+	Err          error
+	SavedRequest *http.Request
+	RequestCount int
+}
+
+func (c *FakeHttpClient) Do(req *http.Request) (*http.Response, error) {
+	c.RequestCount++
+	c.SavedRequest = req
+	if c.Err != nil {
+		return nil, c.Err
+	}
+	return &c.Res, nil
+}