Rework domain errs to be strings

At @goller's recommendation, we're following @benbjohnson's advice laid
out here: https://github.com/benbjohnson/wtf/blob/master/errors.go and
moving errors to the top level package. This cleaned up some ugliness in
the tests where we needed to type assert.

errors.go

@@ -0,0 +1,13 @@
+package mrfusion
+
+// General errors.
+const (
+	ErrUpstreamTimeout = Error("request to backend timed out")
+)
+
+// Error is a domain error encountered while processing mrfusion requests
+type Error string
+
+func (e Error) Error() string {
+	return string(e)
+}

influx/errors.go

@@ -1,13 +0,0 @@
-package influx
-
-const (
-	ErrTimedOut string = "Timeout during request to InfluxDB"
-)
-
-// TimeoutError is returned when requests to InfluxDB are cancelled or time
-// out.
-type TimeoutError struct{}
-
-func (te TimeoutError) Error() string {
-	return ErrTimedOut
-}

influx/influx.go

@@ -45,7 +45,7 @@ func (c *Client) Query(ctx context.Context, query mrfusion.Query) (mrfusion.Resp
 	case resp := <-resps:
 		return resp, resp.err
 	case <-ctx.Done():
-		return nil, TimeoutError{}
+		return nil, mrfusion.ErrUpstreamTimeout
 	}
 }
 

influx/influx_test.go

@@ -79,7 +79,7 @@ func Test_Influx_CancelsInFlightRequests(t *testing.T) {
 	}
 
 	err := <-errs
-	if _, ok := err.(influx.TimeoutError); !ok {
-		t.Error("Expected TimeoutError but wasn't. err was", err)
+	if err != mrfusion.ErrUpstreamTimeout {
+		t.Error("Expected timeout error but wasn't. err was", err)
 	}
 }