slicing cubes that don't have a proper db equivalent from datamodel should fail earlier

[havok2063]

Describe the bug Cubes that are being accessed via the db fail when slicing and the datamodel value does not have a valid entry in the database. For example, slicing MPL-4 cubes to grab spaxels and the spectral_resolution is not present in the database. It fails deep in the Spectrum Quantity generation with a TypeError: The value must be a valid Python or Numpy numeric type. error. I don't think the release matters here but that's how I uncovered it.

To Reproduce

1. delete a spectral resolution array in the database for a specific cube

from marvin import config
config.setRelease('MPL-4')
from marvin.tools import Cube
cube = Cube('8485-1901')
cube[17,17]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-45-02b2682b5165> in <module>()
----> 1 cube[17,17]

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/cube.pyc in __getitem__(self, xy)
    124         """Returns the spaxel for ``(x, y)``"""
    125
--> 126         return self.getSpaxel(x=xy[1], y=xy[0], xyorig='lower')
    127
    128     @staticmethod

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/cube.pyc in getSpaxel(self, x, y, ra, dec, properties, models, **kwargs)
    575                                                       cube=self,
    576                                                       maps=properties,
--> 577                                                       modelcube=models, **kwargs)
    578
    579     def getRSS(self):

/Users/Brian/Work/github_projects/Marvin/python/marvin/utils/general/general.pyc in getSpaxel(cube, maps, modelcube, x, y, ra, dec, xyorig, **kwargs)
    204         _spaxels.append(
    205             marvin.tools.spaxel.Spaxel(jCube[0][ii], iCube[0][ii],
--> 206                                        cube=cube, maps=maps, modelcube=modelcube, **kwargs))
    207
    208     if len(_spaxels) == 1 and isScalar:

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/spaxel.pyc in __init__(self, x, y, cube, maps, modelcube, lazy, **kwargs)
    140
    141         if lazy is False:
--> 142             self.load()
    143
    144     def __dir__(self):

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/spaxel.pyc in load(self, force)
    281
    282         for tool in ['cube', 'maps', 'modelcube']:
--> 283             self._load_tool(tool, force=(force is not None and force == tool))
    284
    285         self._set_radec()

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/spaxel.pyc in _load_tool(self, tool, force)
    327         setattr(self, quantities_dict,
    328                 getattr(getattr(self, '_' + tool), '_get_spaxel_quantities')(self.x, self.y,
--> 329                                                                              spaxel=self))
    330
    331     def getCube(self):

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/cube.pyc in _get_spaxel_quantities(self, x, y, spaxel)
    496                                                     wavelength=self._wavelength,
    497                                                     unit=dm.unit,
--> 498                                                     pixmask_flag=dm.pixmask_flag)
    499
    500         if self.data_origin == 'api':

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/quantities/spectrum.pyc in __new__(cls, flux, wavelength, scale, unit, wavelength_unit, ivar, std, mask, dtype, copy, pixmask_flag, **kwargs)
     64             unit = CompositeUnit(unit.scale * scale, unit.bases, unit.powers)
     65
---> 66         obj = Quantity(flux, unit=unit, dtype=dtype, copy=copy)
     67         obj = obj.view(cls)
     68         obj._set_unit(unit)

/Users/Brian/anaconda2/lib/python2.7/site-packages/astropy/units/quantity.pyc in __new__(cls, value, unit, dtype, copy, order, subok, ndmin)
    379                  isinstance(value.item(() if value.ndim == 0 else 0),
    380                             numbers.Number))):
--> 381             raise TypeError("The value must be a valid Python or "
    382                             "Numpy numeric type.")
    383

TypeError: The value must be a valid Python or Numpy numeric type.

Expected behavior When looping over the datamodel in _get_spaxel_quantities we could probably perform a check to ensure a valid database entry exists. If it's not critical to the Spectrum initialization, issue a warning and generate a Spectrum or return some kind of null value.

Additional context Add any other context about the problem here.