Improve CSS validator error messages

This improves the error messages generated by the CSS validator.

Potential fix for #484. CC @dgmstuart

Example for `h1 { coxlor: rxed; }`:

```
line 1: Property coxlor doesn't exist: h1 { coxlor: rxed; }
```

Contrast this with before:

```
Property coxlor doesn't exist
```

Example for `h1 { ; {`:

```
line 1: Parse Error: h1 { ; {
```

Contrast this with before:

```
Parse Error
```