handle timeout error on sum_bucket/2

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.
```