Quantity's _repr_latex_ does not work on arrays

Currently, one cannot get a latex representation of `Quantity` arrays:
```
import astropy.units as u
q = u.Quantity(1e14, 'm')
q._repr_latex_()
# '$1\\times 10^{+14} \\; \\mathrm{m}$'
q = u.Quantity([1e14, 1.1e14], 'm')
q._repr_latex_()
# ERROR: DeprecationWarning: object.__format__ with a non-empty format string is deprecated [astropy.units.quantity]
```
Obviously, the error message is rather uninformative here!

We should either raise a more informative message, or make arrays work (not sure the latter is either feasible or very useful).