Unless target file exists, exit without serving

I think, if non-existing file is specified as target, `hoi` should not serve that.
So I've added the code that checks the existence of target file.

After this, sample of command line log is as following:

```
% ls hoi.go
hoi.go
% hoi hoi.go; echo $?
http://192.168.1.154:8081/3agqsay199k5w7mhp4nhtquf69y091jl/hoi.go
0
% ls foobar
ls: foobar: No such file or directory
% hoi foobar; echo $?
stat /Users/issm/sandbox/go/hoi/foobar: no such file or directory
1
% tree ~/.hoi
/Users/issm/.hoi
├── conf.json
└── temp_public
    └── 3agqsay199k5w7mhp4nhtquf69y091jl
        └── hoi.go -> /Users/issm/sandbox/go/hoi/hoi.go

2 directories, 2 files
```

`hoi` does not make link of `foobar`, non-existing.