Add helpful information for why module cannot be uninstalled

[artfulrobot]

When trying to uninstall a module you get a message if you can't because other modules depend on it, or because a field uses it. But you're not told which one. As this info is not listed anywhere else, it's quite hard to find out why.

This PR simply adds to the explanation text:

• the list of modules for which the module-to-be-uninstalled is a dependency.
• the list of the (first) field that uses the module.

[poljpocket]

Also, you should always request a PR against the dev branch of the repo. You can find more 'rules' like this here: https://github.com/processwire/processwire/blob/master/CONTRIBUTING.md

[artfulrobot]

@poljpocket Took an embarrassing number of tries to get the formatting correct, but I think I got there in the end! :laughing:

• rebased on dev.
• translate the sprintf format string, not the formatted string.