tor-chanmgr: Do not allocate an Internal error unless we mean it.
Previously we'd allocate an error as a place-holder here, but it's
not a great idea to do that with a Bug
: each Bug
stores a whole
stack trace, which uses a whole pile of allocations to construct.
Now we keep an Option<Error>
instead.
Found while heap profiling.
Closes #383 (closed).