Improve error messages

[spencerkclark]

I have come to notice that minor mistakes in the setup of one's object library can lead to pretty inscrutable error messages, which make it difficult for new users to figure out what might have gone wrong (frequently diagnosing the issue requires knowledge of aospy's internals). In many cases, it shouldn't have to be this way.

Two that I've recently come across are:

• Having a calculation function that returns a numpy array rather than a DataArray; this should be easy to validate (and we can raise an informative error in this instance)
• Having a DataLoader that points to files that do not exist; we should print out what file names the DataLoader was looking for so that the user can try and correct say a typo in their DataLoader definition.

I'm guessing there are others, which we can add to this list as they come up.

[spencerahill]

Totally agree. #136 is related for DataLoaders.

[spencerahill]

#277 addressed the second of these for GFDLDataLoaders