Fix `case_clause` when got `missing_target` error
When mem3 tries to get or create a database, it throws a `missing_target`
error, but it's essentially a `database_does_not_exist` error. So convert
it back in fabric_rpc.erl and try to fix the case_clause error if it occurs.
Error log:
```log
{{case_clause,{error,{error,missing_target,[{mem3_util,get_or_create_db_int,2,[{file,"src/mem3_util.erl"},{line,627}]},{fabric_rpc,all_docs,3,[{file,"src/fabric_rpc.erl"},{line,157}]},{rexi_server,init_p,3,[{file,"src/rexi_server.erl"},{line,141}]}]}}},[{ken_server,update_db_indexes,2,[{file,"src/ken_server.erl"},{line,270}]}]}#012
```
2 files changed
