Changed sim.trange() to use arange instead of linspace

This more accurately reflects the actual simulation timesteps.  E.g.
```python
sim = nengo.Simulator(dt=0.5)
sim.run(2.0) # runs at 4 timesteps: 0.0, 0.5, 1.0, 1.5
sim.trange() # prints 0.0, 0.66, 1.33, 2.0
```
Basically linspace tries to space things evenly, whereas arange gives you the actual `dt` intervals.

Not a very critical change, just thought I'd get it in there before I forget.

I'm also sneaking in a small change to the error returned when you get a TypeError from a Node function.  It's helpful to get the actual TypeError, since it is very possible that your Node function throws a TypeError unrelated to argument dimensions.  Seemed silly to make a 4 character change its own PR though.