Use cached template if template download fails

[BraulioVM]

I saw that issue #3850 was newcomer friendly and so I'm trying to fix it.

I intend to add tests once I make sure I'm doing things properly. For the time being, I have manually tested the feature by:

1. Properly downloading a valid template $ stack new tmp https://raw.githubusercontent.com/stesta/haskell-templates/master/snap-basic-websockets.hsfiles
2. Removing the tmp directory: $ rm -r tmp.
3. Disabling network connectivity on my computer.
4. Trying to use the template again by executing the command in 1 again.

And it works as I intended. I haven't tried yet with a TemplatePath using the RepoPath constructor but that should be quick to try. I haven't seen what would happen if redownload throws a
RedownloadInvalidResponse instead of a RedownloadHttpError either, but the worst thing that I could see happening is a log error badly formatted. I'll take a look at that case too anyway.

I have tried to make sure nobody was catching a HttpException when calling redownload from any part of the codebase. That could be slightly problematic because, as I'm catching that exception myself at redownload, it would never propagate to the former HttpException catchers, which could have been doing something useful with the exception. I've grepped redownload and there is no function calling it that catches the exception, but I can't be sure about functions higher in the call stack. I would say this is not going to be a problem, but wanted to find out what you think about it.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

[mihaimaruseac]

Is this file needed somewhere?

[BraulioVM]

No! I left that by accident because I needed it for the first implementation of the integration test. I'll remove it.

[mihaimaruseac]

Thanks, looks really good, except that question on the simple.hsfiles template

[BraulioVM]

I feel like I could do some more testing (trying with a RepoPath for example, which will be easy). I will do that tonight (in like 6-7 hours).

I believe I should add this to the ChangeLog.md. What about the docs?

[mihaimaruseac]

The only relevant place in the docs on a quick scan seems to be https://docs.haskellstack.org/en/stable/GUIDE/#stack-new but even there it would be offtopic to talk about using cached templates.

[BraulioVM]

I stumbled upon some issues that made stack build the whole project every time I made a change in the tests, so I needed more time than expected to finish the PR. I'm not sure what it was but I fixed it by removing the ~/.stack directory.

Anyway, tell me if there's anything missing. I will try to be quick.

[mihaimaruseac]

Looks good, thanks