Fix length of error marker for missing src file

The length was not calculated properly. Length of attr.getValue() was
taken while it should be the length of attr.getValueRegionText(). The
difference comes when the attribute value is wrapped in quotes.

The problem is especially visible when using the dark theme.
![image](https://cloud.githubusercontent.com/assets/468091/5073611/c435b426-6e8b-11e4-988c-e28aa8ff9a18.png)