Update fused kernel error recommendation, and make error logs more verbose

[Quentin-Anthony]

This PR makes two small changes:

• Sometimes the python megatron/fused-kernels/setup.py install will fail with errors that pip install megatron/fused-kernels will not cause (e.g. a nasty undefined torch symbol that I encountered today). I therefore updated the error message to recommend pip install instead.
• Failing to import a fused kernel can be due to many issues (i.e. broken install) besides the kernels not being installed, so we should print the specific error as well.