handle timeout error on sum_bucket/2

[kuenishi]

addresses #743.
from commit message:

add error handling when sum_bucket failed with timeout, which
causes dirty error message and no way to know which bucket was
failed in usage calculation mapreduce. This commit adds two ways
to know that:

- outputs an error log that prints user key id and bucket name
  with error reason.
- stores the result into moss.storage with reason like:
    { "bucket_name" : "{error,{timeout,[]}}" }

The latter log proves and indicated there existed a bucket at
that time even though its usage size was unknown. Better than
ignored, which operator can't tell non-existent from calculation
failure.

[andrewjstone]

Code looks good.

• cs743_regression_test riak_test
• eunit + eqc
• dialyzer
• xref

[andrewjstone]

I'm seeing the following dialyzer errors related to this change:

riak_cs_storage.erl:60: Invalid type specification for function riak_cs_storage:maybe_sum_bucket/3. The success typing is (pid(),[binary() | maybe_improper_list(any(),binary() | []) | byte()],binary() | [byte()]) -> {binary() | [byte()],binary()}
riak_cs_storage.erl:64: The pattern {'ok', BucketUsage} can never match the type {'error',_} | {'struct',[{_,_},...]}

[kuenishi]

Hooray! It was totally a bug found by dialyzer.
This regression could be found if #703 were done.

[andrewjstone]

👍

[reiddraper]

s/unless/without

[kuenishi]

Oh crap. Thank you for pointing it out.