Fix the following dialyzer errors

[andrewjstone]

We aren't checking return values for these which implies we should just match on ok and fail fast.

dialyzer -Wno_return -Wunmatched_returns -Wrace_conditions \
        --plt /Users/ajs/.eleveldb_dialyzer_plt ebin | tee .dialyzer.raw-output
  Checking whether the PLT /Users/ajs/.eleveldb_dialyzer_plt is up-to-date... yes
  Proceeding with analysis...
eleveldb.erl:126: Expression produces a value of type {'error',_} | {'ok',eleveldb:db_ref()}, but this value is unmatched
eleveldb.erl:156: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
eleveldb.erl:174: Expression produces a value of type {_,'ok',eleveldb:itr_ref()}, but this value is unmatched
eleveldb.erl:180: Expression produces a value of type {_,'ok',eleveldb:itr_ref()}, but this value is unmatched
 done in 0m1.24s
done (warnings were emitted)

[evanmcc]

@andrewjstone have these been fixed?

[andrewjstone]

I don't see them anymore when running dialyzer on develop, so I suppose this can be closed.