Print workaround on ros_adapter_test import error

Having a locally installed google-auth Python package may cause bazel
test to fail, a consequence of bazel's Python rules not being hermetic.

Users following our how-to guides may run into this, as they are asked
to pip install google-auth.

See also:
bazelbuild/bazel#890 (comment)

Change-Id: Idc0d34ecc81b910b663bf380da14c3e7cf6d8415

src/python/ros_adapter/ros_adapter_test.py

@@ -14,7 +14,12 @@
 
 """Tests for ros_adapter."""
 
-import ros_adapter
+try:
+  import ros_adapter
+except ImportError as e:
+  print "ImportError possibly caused by locally installed Python packages"
+  print "try: pip uninstall google-auth"
+  raise e
 
 
 if __name__ == '__main__':