Better error if git not installed

[itowlson]

Fixes #953.

The output when git is not installed is still not 100% crisp, and we could do with bypassing the context stack for these "known" cases. But this at least gives the specific message pointing the finger at git:

# --- Template case ---
$ spin templates install --git https://github.com/fermyon/spin
Copying remote template source
Error: Failed to install one or more templates

Caused by:
    0: Failed to get template source
    1: Error cloning Git repo https://github.com/fermyon/spin: `git` command not found - is git installed?

# --- Plugin case ---
$ spin plugin install js2wasm
Error: failed to connect to endpoint https://github.com/fermyon/spin-plugins/: Error cloning Git repo https://github.com/fermyon/spin-plugins/: `git` command not found - is git installed?

I'm not sure there is a sensible way to deduplicate this with the current cratification of the Spin CLI, bar lots of micro-crates or the spin-util crate of shame, but I'm open to ideas!

Signed-off-by: itowlson ivan.towlson@fermyon.com

[kate-goldenring]

For deduplication, should we create a single git crate that both templates and plugins use, containing the pulling and error capabilities?

[itowlson]

@kate-goldenring We could create a git crate. I worry, though, that whole crates for each relatively small piece of duplication is going to get fiddly to manage. Although having an entry called sloth in the folder list would certainly cheer me up in the mornings.

[kate-goldenring]

@itowlson it seems like this could be a larger discussion we could have around what the criteria for utility crates are in spin. We could leave duplication for now and create an issue around the discussion?

[itowlson]

@kate-goldenring I opened #965 but yeah, it would be nice not to have to wait on that discussion!