Return ErrNotFound if value is not found

I modified functions to return the fixed error `ErrNotFound`.
It let us know whether `git config` unexpectedly aborted or just entry not found as following:
```go
v, err := gitconfig.Global("diff.tool")
if err == gitconfig.ErrNotFound {
    println("diff.tool not configured, use /usr/bin/diff command")
    v = "/usr/bin/diff"
}
```