Remove duplicate NotFoundError and rename it to ErrNotFound (idiomatic)

Error had to be defined in its own package to avoid import cycles.

Author: toqueteos

client.go

@@ -7,6 +7,7 @@ import (
 	"net/url"
 	"strings"
 
+	"github.com/tyba/srcd-crawler/clients/common"
 	"github.com/tyba/srcd-crawler/clients/git/pktline"
 
 	"github.com/sourcegraph/go-vcsurl"
@@ -35,7 +36,7 @@ func (c *Client) Refs() (*Refs, error) {
 	}
 
 	if res.StatusCode >= 400 {
-		return nil, &NotFoundError{c.url}
+		return nil, &common.ErrNotFound{c.url}
 	}
 
 	defer res.Body.Close()
@@ -150,14 +151,6 @@ func (c *Client) applyHeadersToRequest(req *http.Request, content *strings.Reade
 	}
 }
 
-type NotFoundError struct {
-	url string
-}
-
-func (e NotFoundError) Error() string {
-	return e.url
-}
-
 type Refs struct {
 	defaultBranch string
 	branches      map[string]string