Use exception chaining when re-raising numpy exceptions within BoundedArray.__init__

[cool-RR]

I recently went over Matplotlib, Pandas and NumPy, fixing a small mistake in the way that Python 3's exception chaining is used. I found the same mistake here in two places, and fixed it in this PR.

The mistake is this: An exception is being caught and replaced with a more user-friendly error. In these cases the syntax raise new_error from old_error needs to be used.

Python 3's exception chaining means it shows not only the traceback of the current exception, but that of the original exception (and possibly more.) This is regardless of raise from. The usage of raise from tells Python to put a more accurate message between the tracebacks. Instead of this:

During handling of the above exception, another exception occurred:

You'll get this:

The above exception was the direct cause of the following exception:

The first is inaccurate, because it signifies a bug in the exception-handling code itself, which is a separate situation than wrapping an exception.

Let me know what you think!

[alimuldal]

Thanks - this is basically a hangover from the fact that we needed to support Python 2 until recently.

I don't think there's any need to include the message from the original exception in the new exception message, since the original exception will still visible in the traceback. I'd just do:

except ValueError as numpy_exception:
  raise ValueError('`minimum` is incompatible with `shape`') from numpy_exception

[cool-RR]

Agreed, fixed.

[alimuldal]

Merged in I9d16586a559d938d608afcaf06500ddc92adb209. Thanks!