Avoid silently throwing error
Avoid silently throwing error

With `throw`, tests of sumo_db_riak were failing locally for me with
the following error:

```
%%% basic_SUITE ==> find: SKIPPED
%%% basic_SUITE ==> {tc_auto_skip,
    {auto_skipped,{failed,{basic_SUITE,init_per_testcase,{error,0}}}}}
```

that does not provide information on the cause.

With `exit`, the same tests fail with:

```
%%% basic_SUITE ==> find: SKIPPED
%%% basic_SUITE ==> {tc_auto_skip,
    {auto_skipped,
        {failed,
            {basic_SUITE,init_per_testcase,
                {{error,0},
                 [{sumo,delete_all,1,
                      [{file,
                           "/Users/luca/dev/vuk/inaka_crud/sumo_db_riak/_build/default/lib/sumo_db/src/sumo.erl"},
                       {line,227}]},
                  {sumo_basic_test_helper,init_store,1,
                      [{file,
                           "/Users/luca/dev/vuk/inaka_crud/sumo_db_riak/_build/default/lib/sumo_db/src/adapter_test_helpers/sumo_basic_test_helper.erl"},
                       {line,165}]},
                  {basic_SUITE,init_per_testcase,2,
                      [{file,
                           "/Users/luca/dev/vuk/inaka_crud/sumo_db_riak/_build/test/lib/sumo_db_riak/test/basic_SUITE.erl"},
                       {line,51}]},
                  {test_server,do_init_per_testcase,2,
                      [{file,"test_server.erl"},{line,1297}]},
                  {test_server,run_test_case_eval1,6,
                      [{file,"test_server.erl"},{line,1007}]},
                  {test_server,run_test_case_eval,9,
                      [{file,"test_server.erl"},{line,977}]}]}}}}}
```

See also:
* https://github.com/inaka/erlang_guidelines/pull/70
* https://github.com/inaka/erlang_guidelines/pull/71

inaka/sumo_db