Better error message when file not found

[egrimley-arm]

If the parsec binary finds that /tmp/parsec is missing it emits the error

Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

If the parsec binary does not find ./config.toml it emits the same error

Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

In both cases it would be helpful to mention which file or directory it was looking for (even though strace can answer that question quite quickly).

[ionut-arm]

I believe the /tmp/parsec problem is solved by #205 - I'll raise a PR shortly to make the error message for the config clearer.