Add "unknown advisory" detection

[senden9]

Fixes #64.
Warn if a advisory should be ignored that does not exist at all.

Example:

➜  test2 git:(master) ✗ ~/git/cargo-deny/target/debug/cargo-deny check advisories
warning: this advisory is not in any RustSec database
   ┌─ /tmp/test2/deny.toml:49:5
   │
49 │     "RUSTSEC-0000-0000",
   │     ^^^^^^^^^^^^^^^^^^^ unknown advisory

warning: advisory was not encountered
   ┌─ /tmp/test2/deny.toml:48:5
   │
48 │     "RUSTSEC-2020-0036",
   │     ^^^^^^^^^^^^^^^^^^^ no crate matched advisory criteria

advisories ok

[Jake-Shadle]

Nice, thanks!