gracefully handle git secrets not being installed

[lukaso]

Issue #, if available:

Description of changes:

For whatever reason, sometimes the env does not have access to git secrets (for example in vscode this seems to happen frequently). This change allows it to fail gracefully instead of blocking all commits.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sparr]

Suggested change

	echo "err=\"\$(git secrets --${cmd} -- \"\$@\" 2>&1 > /dev/null)\"" >> "${dest}"
	echo "if [[ \"\$err\" == *\"git: 'secrets' is not a git command\"* ]]; then" >> "${dest}"
	echo " exit 0" >> "${dest}"
	echo "elif [[ ! -z \"\$err\" ]]; then" >> "${dest}"
	echo " >&2 echo \"\$err\"" >> "${dest}"
	echo " exit 1" >> "${dest}"
	echo "fi" >> "${dest}"
	cat <<-EOF > "${dest}"
	err="\$(git secrets --${cmd} -- "\$@" 2>&1 > /dev/null)"
	if [[ "\$err" == *"git: 'secrets' is not a git command"* ]]; then
	exit 0
	elif [[ ! -z "\$err" ]]; then
	>&2 echo "\$err"
	exit 1
	fi
	EOF

I think this will be easier to read, but it needs to be tested.
Also, due to github limitations, this suggestion doesn't replace the first echo on line 205 but should.