Exit gracefully from hook scripts when requisite bins missing

[calebcartwright]

We're deliberate in the underlying git hook scripts to avoid failing (and thus breaking git flows) in environmental cases, such as a missing configuration file, but there's a big case we've missed entirely

rusty-hook/src/hook_files/cli.sh

Lines 47 to 57 in e284e15

	if [ ${rustyHookExitCode} -eq ${noConfigFileExitCode} ]; then
	if [ "${hookName}" = "pre-commit" ]; then
	echo "[rusty-hook] rusty-hook git hooks are configured, but no config file was found"
	echo "[rusty-hook] In order to use rusty-hook, your project must have a config file"
	echo "[rusty-hook] See https://github.com/swellaby/rusty-hook#configure for more information about configuring rusty-hook"
	echo
	echo "[rusty-hook] If you were trying to remove rusty-hook, then you should also delete the git hook files to remove this warning"
	echo "[rusty-hook] See https://github.com/swellaby/rusty-hook#removing-rusty-hook for more information about removing rusty-hook from your project"
	echo
	fi
	exit 0

However, we've overlooked the scenarios where rusty-hook bin is not already on the system and the subsequent cli installation fails, as well as cargo itself not being available. These are not impossible scenarios by any stretch of the imagination, and could occur relatively easily, for example if cargo's bin directory or cargo itself is not on the user's path, and we've had a couple reports (albeit with different causes) of hook issues due to the executables not being found (refs #105 #140).

To prevent this problematic behavior, we should:

• check for cargo availability before attempting to use it and exit with a message should it not be (https://github.com/swellaby/rusty-hook/blob/master/src/hook_files/cli.sh)
• check for rusty-hook availability/installation success instead of just assuming the install succeeded and is available in cases where it wasn't already on thte system
  https://github.com/swellaby/rusty-hook/blob/master/src/hook_files/hook_script.sh#L13