Raise more informative error when interp1d is given unsorted x

Right now the error is confusing.

```python
from scipy.interpolate import interp1d
x = [0.1, 0.2, 0.]
y = [0.5, 0.6, 0.4]
f = interp1d(x, y)
print f(0.)
```
results in:

    ValueError: A value in x_new is below the interpolation range.