Handle non-installed valgrind gracefully

[jfrimmel]

In the 2.0.0 version of this crate, there is a plain panic, if valgrind is not installed or can otherwise not be run This is due to this lines (the .expect()):

cargo-valgrind/src/main.rs

Lines 59 to 64 in 2da5213

	let mut cargo = Command::new("valgrind")
	.arg("--xml=yes")
	.arg(format!("--xml-socket={}:{}", address.ip(), address.port()))
	.args(command)
	.spawn()
	.expect("Valgrind is not installed or cannot be started");

It would be better to gracefully handle that case and print a user error, optionally with install instructions, e.g.:

$ cargo valgrind run; echo $?
error: `valgrind` is not installed, please install valgrind via your package manager.
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