Don't throw errors when HAL emits a warning

[virtuald]

• Fixes #143

To test this, you can do the following:

git clone https://github.com/robotpy/robotpy-wpilib.git
cd robotpy-wpilib
git checkout hal-warning

On a linux/OSX system, or if you have MSYS installed on windows, you can run devtools/build_and_install.sh. If you don't have either of those, post a comment and I can write something up that will get you going.

[arilotter]

Seems to have worked for me! I deployed it and it's not crashing anymore. On the other hand, I'm not getting any HAL warnings, so the problem might just have fixed itself (CAN is weird sometimes)

[arilotter]

Even better! Just got a warning.

/usr/local/lib/python3.4/site-packages/wpilib/powerdistributionpanel.py:56: UserWarning: CTRE CAN Recieve Timeout
  return hal.getPDPTotalCurrent()

Works perfectly, thanks for the patch :)