Updating error message for locate() methods.

[dhermes]

Also adding a note that locate() / evaluate*() are (essentially) inverses.

See #35 for context.

/cc @pdknsk

Old error:

>>> import numpy as np
>>> import bezier
>>> curve = bezier.Curve.from_nodes(np.asfortranarray([
...     [0.0, 0.0],
...     [0.5, 0.0],
...     [0.5, 1.0],
...     [1.0, 1.0],
... ]), _copy=False)
>>> curve.evaluate(0.5)
array([[ 0.5,  0.5]])
>>> curve.locate(np.asfortranarray([[0.5]]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../bezier/curve.py", line 898, in locate
    point, 'Shape expected:', (1, self._dimension))
ValueError: ('Point is not in same dimension as curve', array([[ 0.5]]), 
             'Shape expected:', (1, 2))

New error:

>>> curve.locate(np.asfortranarray([[0.5]]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../bezier/curve.py", line 908, in locate
    raise ValueError(msg)
ValueError: Dimension mismatch: This curve is 2-dimensional, so the point should be
            a 1x2 NumPy array. Instead the point [[ 0.5]] has dimensions 1 x 1.