Ensure Briefcase can run while offline

[freakboy3742]

As long as you have previously downloaded templates, support packages etc, it should be possible to run Briefcase while you have no internet connection.

At present, being offline raises loud errors.

There may be other online checks that need to be gated.

Two changes are required:

• There should be an --offline command line argument that will silence (or at least radically quieten down) warnings related to being offline
• Any request to access the network should be able to fallback to previously downloaded files.

The NetworkFailure exception may provide a useful way to identify and catch these cases.

[MosGeo]

I'll add to this that the fallback file should be transferable by the user to a clearly defined folder. For example, the user can set it to a custom directory in the toml file.

[freakboy3742]

@MosGeo It's already possible to manually specify templates or support packages as part of your project configuration; the issue is that this won't necessarily prevent network access.

[MosGeo]

I am able to use briefcase in offline mode after copying all the required tools, templates manually and setting the environment variable. I don't have the project on me right now to provide exact info.

Now, I am able to build and package my project completely offline without any warnings.

Note that I am doing this for windows. I have not rested it for Android.

[freakboy3742]

After some time trying, I found a rough idea to be able to run offline: first realize a successful run, then copy the JDK and Android SDK to the project...

Thanks, but that's not really a solution to this problem - it's a (partial) fix for #738.

The issue this ticket describes isn't getting the tools - it's working around the various places that Briefcase (and the tools Briefcase invokes) perform explicit "am I up to date?" checks. These currently require network access. It doesn't matter if you have those tools already - Briefcase is expecting to be able to request updates; those cases need to be either avoided, or the network error caused by being offline needs to be caught.