failing as we should, instead of printing out an error

[vineetbansal]

I'm not sure why the following print statements were put in - merely printing out an error and proceeding like nothing happened doesn't accomplish anything IMO, and only hides the otherwise invaluable stack trace. It's better to fail explicitly with an ImportError.

I noticed this when a bad installation of torch in my environment was preventing an internal import deep inside pytorch (triggered in our code on from torch.nn import Module), but this was hard to track down because of this print.

If the intention here was to allow osqp code execution even in the absence of the torch module, that's understandable, but again, no other code in osqp seems to be inadvertently picking up this module where it shouldn't except a unit test, and doing an from osqp.nn.torch import .. should trigger an ImportError.

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