On run with replace, warn if executable not found

[stefanv]

Closes #270

[oscarbenjamin]

Would it potentially make sense to change it here instead:

spin/spin/cmds/util.py

Lines 67 to 76 in 210ae8e

	if replace and (sys.platform in ("linux", "darwin")):
	os.execvp(cmd[0], cmd)
	print(f"Failed to launch `{cmd}`")
	sys.exit(-1)
	else:
	try:
	p = subprocess.run(cmd, *args, **kwargs)
	except FileNotFoundError:
	click.secho(f"`{cmd[0]}` executable not found. Exiting.", fg="bright_red")
	raise SystemExit(1) from None

[stefanv]

We can do that; I wasn't sure how run might be used in the wild. E.g., someone might already be relying on p = run(...) and then checking the error code. But perhaps fine.

[stefanv]

Ah, OK, I see we already do it for one branch, so then, yes, definitely.

[stefanv]

OK, try now

[oscarbenjamin]

Looks good to me. Thanks!