Error is too vague when rustc is missing

[glandium]

When rustc is not found, this is the error that is printed:

[ERROR] Error while executing llvm tools: No such file or directory (os error 2)

That comes from

grcov/src/llvm_tools.rs

Line 153 in 9cc638b

let output = Command::new(rustc).arg("--print").arg("sysroot").output()?;

that fails with an std::io::Error that doesn't contain information about what specifically failed.

[glandium]

It could be argued that it could fallback to trying to find llvm tools in $PATH, which it also doesn't do.