Importing chaco without X server running causes a crash.

**Problem Description**
Trying to import chaco without X server running on Linux causes python to terminate.

**Reproduction Steps:**
1. Ensure that the DISPLAY environment variable has been unset in the terminal.
`unset DISPLAY`
2. Start the Python3.6 interpreter and run
`import chaco.api`

```python
import chaco.api
```

**Expected behavior:**
Import should not cause a crash.
This is important for example, when using chaco on a remote server without X server running.
Error on trying to display something is acceptable but not a crash when importing.

**OS, Python version:** Linux, Python3.6