Improve error message when refusing to remove image due to multiple repo tags

docs/sources/commandline/cli.rst

@@ -952,7 +952,7 @@ image is removed.
     test2                     latest              fd484f19954f        23 seconds ago      7 B (virtual 4.964 MB)
 
     $ sudo docker rmi fd484f19954f
-    Error: Conflict, fd484f19954f wasn't deleted
+    Error: Conflict, cannot delete image fd484f19954f because it is tagged in multiple repositories
     2013/12/11 05:47:16 Error: failed to remove one or more images
 
     $ sudo docker rmi test1

server.go

@@ -1567,7 +1567,7 @@ func (srv *Server) deleteImage(img *Image, repoName, tag string) ([]APIRmi, erro
 			} else if repoName != parsedRepo {
 				// the id belongs to multiple repos, like base:latest and user:test,
 				// in that case return conflict
-				return imgs, nil
+				return nil, fmt.Errorf("Conflict, cannot delete image %s because it is tagged in multiple repositories", utils.TruncateID(img.ID))
 			}
 		}
 	} else {