Sysno::from_str should return well error type

See https://rust-lang.github.io/api-guidelines/interoperability.html#c-good-err

I would recommend a type like the following:

```rust
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
struct ParseSysnoError {
string: String,
}
impl fmt::Display for ParseSysnoError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "provided string '{}' was not a known syscall", &self.string)?;

Ok(())
}
}
impl std::error::Error for ParseSysnoError {}
```

Locate the implementation of Sysno::from_str and any existing parsing tests. Compare its current error behavior with the proposed ParseSysnoError shape and the Rust API guideline linked in the issue. Done means the method returns a well-defined error containing the provided string, with tests covering the failure case.

Written by the indexing model from the issue text.