Catch TraitError when trying to set line_width.

Trying to run the imshow example from the mayavi documentation a TraitError is thrown trying to set the line_width. This seems to be because Mayavi expects an AttributeError where a TraitError is thrown. This fixes that by also catching TraitErrors. 

```python
import numpy
from mayavi.mlab import *

def test_imshow():
    """ Use imshow to visualize a 2D 10x10 random array.
    """
    s = numpy.random.random((10,10))
    return imshow(s, colormap='gist_earth')
```