Flutter clean should use noExitOnFailure when deleting

[zanderso]

Currently when deleting, the EHFS will throwToolExit eagerly when deletion fails due to an access error. However the implementation of the clean command has its own logic for handling a failure to delete. This means users will likely never see the potentially helpful message here: https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/clean.dart#L113.

Instead of deletion is called under EHFS.noExitOnFailure, the EHFS will throw an Exception instead of throwToolExit, which can then be caught by the clean command, which can still print its own message.

An alternative would be to take the advice from the clean command and add it to the advice in the EHFS when deletion fails due to an access error on Windows.

cc @christopherfujino