Fix error handling.

[stereosteve]

Running specs on master w/ 0.21.1 I got:

Invalid memory access (signal 11) at address 0x0
[4459433179] *CallStack::print_backtrace:Int32 +107
[4459379463] __crystal_sigfault_handler +55
[140736114596666] _sigtramp +26
[4460655070] _Z9SaveErrorPPcRKN7leveldb6StatusE +26
[4460654944] leveldb_open +96
[4459785091] *LevelDB::DB#initialize<String>:Bool +211
[4459784857] *LevelDB::DB::new<String>:LevelDB::DB +137
[4459389083] ~procProc(Nil)@spec/leveldb/db_spec.cr:5 +571
[4459392043] *it<String, String, Int32, Int32, &Proc(Nil)>:(Array(Spec::Result) | Nil) +379
[4459388147] ~procProc(Nil)@spec/leveldb/db_spec.cr:4 +115
[4459670866] *Spec::RootContext::describe<String, String, Int32, &Proc(Nil)>:Spec::Context+ +354
[4459393593] *describe<String, String, Int32, &Proc(Nil)>:Spec::Context+ +9
[4459388011] ~procProc(Nil)@spec/leveldb/db_spec.cr:3 +107
[4459670866] *Spec::RootContext::describe<String, String, Int32, &Proc(Nil)>:Spec::Context+ +354
[4459403059] *describe<LevelDB:Module, String, Int32, &Proc(Nil)>:Spec::Context+ +51
[4459314439] __crystal_main +3543
[4459379208] main +40

Based on advice from jhass in the chat and looking at
levedb c api
converted to use
out param.
Specs pass now.

[veelenga]

Sorry, it seems to me this PR is out of date. If I am wrong, please rebase or prepare a new PR and I will be happy to review/merge.