Eliminate ErrorKind::TODO

This variant needs to be eliminated before we can call our error conversion complete.

We can probably do this one crate at a time. We should probably also refactor our errors as we go.

These crates are low-level, and might not all need to provide ErrorKind.

• caret (no errors.)
• retry-error (declares an error type wrapper; can't have a kind).
• tor-error (already done.)
• tor-config (has multiple errors.) !296 (merged)
• tor-events (too low-level, and not yet used.)
• tor-units (too low-level)
• tor-rtcompat (uses IoError everywhere; probably needs refactoring)
• tor-rtmock (testing-only; should not declare user-visible errors)
• tor-llcrypto (too low-level)
• tor-protover (too low-level)
• tor-bytes (too low-level, but can use InternalError) !300 (merged)
• tor-socksproto !299 (merged)
• tor-checkable (too low-level)
• tor-cert (declares no error types)
• tor-linkspec (declares no error types)
• tor-cell !304 (merged)

(Everything here and below will probably have ErrorKinds)

• tor-proto !304 (merged)
• tor-netdoc refactored in !314 (merged)
• tor-consdiff (no kinds needed here)
• tor-netdir (no kinds needed here, but refactoring will help) !319 (merged)
• tor-persist (already done)
• tor-chanmgr !324 (merged)
• tor-guardmgr (Already done)
• tor-circmgr !325 (closed)
• tor-dirclient !325 (closed)
• tor-dirmgr !325 (closed)
• arti-client !325 (closed)
• arti-config (tbd?)
• arti-bench (No public API)
• arti (No public API (yet?))