config: check config names more carefully and report errors - fixes #3506

Before this change it was possible to make a remote with an invalid
name in the config file, either manually or with `rclone config
create` (but not with `rclone config`).

When this remote was used, because it was invalid, rclone would
presume this remote name was a local directory for a very suprising
user experience!

This change checks remote names more carefully and returns errors
- when the user tries to use an invalid remote name on the command line
- when an invalid remote name is used in `rclone config create/update/password`
- when the user tries to enter an invalid remote name in `rclone config`

This does not prevent the user entering a remote name with invalid
characters in the config manually, but such a remote will fail
immediately when it is used on the command line.

fs/fs.go

@@ -1087,7 +1087,10 @@ func MustFind(name string) *RegInfo {
 // ParseRemote deconstructs a path into configName, fsPath, looking up
 // the fsName in the config file (returning NotFoundInConfigFile if not found)
 func ParseRemote(path string) (fsInfo *RegInfo, configName, fsPath string, err error) {
-	configName, fsPath = fspath.Parse(path)
+	configName, fsPath, err = fspath.Parse(path)
+	if err != nil {
+		return nil, "", "", err
+	}
 	var fsName string
 	var ok bool
 	if configName != "" {