Make the CLI usable without rustup

[TimJentzsch]

We currently assume the user has rustup installed, but that isn't always the case.
This makes the web sub-commands unusable, because they use rustup to check whether the wasm32-unknown-unknown target is installed.

As a quick fix, we could first check whether rustup is installed and if it's not, we simply omit the Wasm target check.

Later on, I assume we will not check this in advance, but parse the command output for the error instead and then do it on demand.