Handle file not found errors

#### Example Output

```
$ markdown-pdf example.md
events.js:160
throw er; // Unhandled 'error' event
^

Error: ENOENT: no such file or directory, open 'example.md'
at Error (native)
```

Exit status

```
$ echo $?
1
```
#### Desired Output (or something along these lines)

```
$ markdown-pdf example.md
example.md: no such file or directory
```

Exit status

```
$ echo $?
2
```