API client (Go) returns error on 404 rather than nil result

### Nomad version

Output from `nomad version`

0.4.1
### Operating system and Environment details

macOS
### Issue

When using the API client package in the `api` subdir, and making a request such as:

``` go
client.Jobs().Info("foo", nil)
```

where "foo" doesn't exist, it returns an error, rather than a nil job. This makes it difficult to differentiate between a real error and simply a non-existent job.
### Reproduction steps

The above states it.
### Nomad Server logs (if appropriate)

Not appropriate.
### Nomad Client logs (if appropriate)

Not appropriate.
### Job file (if appropriate)

None required.

Start in the API client package under the api subdirectory and trace client.Jobs().Info("foo", nil) for a missing job. Reproduce the 404 case and verify that a nonexistent job produces a nil result without an error, while genuine errors remain distinguishable.

Written by the indexing model from the issue text.