Added meaningful error if retrieving nil val from layer. Fixes #386

[brollb]

WIP #386 Added googlenet-setters for testing error messages

[brollb]

This change makes it so it will throw an error if a value is accessed from a layer that is nil (and it will report the name of the value that was tried to access). This gives us better error messages as it throws a meaningful error before the could not call nil error that was there before.

Unfortunately, this doesn't check that the return value of the get is going to be called. This can be a problem because things like the following will also fail:

nn.Sequential().size = 10

[brollb]

Just updated it so it will throw an error if any unsupported value is accessed in a "non-setting" way.

That is, the following will succeed:

model.imageSize = 256

but the following will fail:

nn.SpatialMaxPooling():ceil()

It is worth noting that print(model.imageSize) will fail if it hasn't been set in the file. This is the desired behavior bc I am assuming that accessed values should exist. If they do not exist in the nn mock, then it is an error if the script relies on using them.