Ensure that quantity operations with incompatible types give a useful TypeError

[mhvk]

Currently, a rather uninformative TypeError is raised if one, e.g., adds a quantity to something that is not a number:

In [1]: import astropy.units as u, numpy as np

In [2]: np.add(np.arange(10.)*u.cm,  u.cm)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-63526e011ceb> in <module>()                                    
----> 1 np.add(np.arange(10.)*u.cm,  u.cm)                                      

/usr/lib/python3/dist-packages/astropy/units/quantity.py in __array_prepare__(self, obj, context)                                                               
    294             # (this allows, e.g., `q > 0.` independent of unit)         
    295             maybe_arbitrary_arg = args[scales.index(0.)]                
--> 296             if _can_have_arbitrary_unit(maybe_arbitrary_arg):           
    297                 scales = [1., 1.]                                       
    298             else:                                                       

/usr/lib/python3/dist-packages/astropy/units/quantity.py in _can_have_arbitrary_unit(value)
     57     `True` if each member is either zero or not finite, `False` otherwise
     58     """
---> 59     return np.all(np.logical_or(np.equal(value, 0.), ~np.isfinite(value)))
     60 
     61 

TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

With this PR, this becomes:

In [2]: np.add(np.arange(10.)*u.cm,  u.cm)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-63526e011ceb> in <module>()
----> 1 np.add(np.arange(10.)*u.cm,  u.cm)

/data/mhvk/venv/astropy-dev/lib/python3.4/site-packages/astropy-1.0.dev9886-py3.4-linux-x86_64.egg/astropy/units/quantity.py in __array_prepare__(self, obj, context)
    310                                 .format(function.__name__,
    311                                         args[0].__class__.__name__,
--> 312                                         args[1].__class__.__name__))
    313 
    314         # In the case of np.power, the unit itself needs to be modified by an

TypeError: Unsupported operand type(s) for add: 'Quantity' and 'Unit'

[astrofrog]

👍 - maybe you could also check the content of the exception in the test, since that is the main change here?

[mhvk]

@astrofrog - that check on the error message turned out to be more of a hassle than thought, but anyway, it is done. I think this is ready now.

[mhvk]

@astrofrog - do you think this is ready to go in? @embray - this just clarifies an exception, so I don't think this necessarily has to be in 0.4.2; would be happy to change it to 0.4.3 if that helps you.

[mhvk]

Rebased and moved the changelog entry to 0.4.3. @astrofrog - I think this is ready to go in (assuming travis still passes), so marked it as such. I also applied the Affects-release tag; @embray - hopefully, this is the right usage?

[embray]

Thanks @mhvk , looks good!