Provide better error messages when ROS_PACKAGE_PATH is undefined.

[chadrockey]

Original author: damonkoh...@google.com (August 08, 2012 05:32:22)

What steps will reproduce the problem?

1. ./gradle install without ROS_PACKAGE_PATH defined
2. NPE from GenerateInterfaces:162

What is the expected output? What do you see instead?
A reasonable error message.

Original issue: http://code.google.com/p/rosjava/issues/detail?id=135

[chadrockey]

From tibokr...@gmail.com on August 08, 2012 10:22:15
See patch in
http://code.google.com/p/rosjava/issues/detail?id=132