[jsk_recognition_utils] use try to import chainer depend modules

[knorth55]

use try to import chainer depending modules.
this PR solves jsk-ros-pkg/jsk_visualization#800

cc. @Naoki-Hiraoka

[Naoki-Hiraoka]

In additon to chainer, I found that jsk_recognition_utils/chainermodels also depends on
fcn

[pazeshun]

I also found jsk_recognition_utils.datasets depends on chainercv:

jsk_recognition/jsk_recognition_utils/python/jsk_recognition_utils/datasets/bbox_detection.py

Line 6 in 2c1400e

from chainercv.chainer_experimental.datasets.sliceable import GetterDataset

I think it is troublesome to list all required packages in the printing message, so how about Please install chainer<7.0.0 and execute "rosdep install jsk_recognition_utils" to import jsk_recognition_utils.chainermodels and datasets?

[knorth55]

@pazeshun I update this branch. I found that chainer, chainercv and fcn are chainer depending packages, so I check these 3 packages. How about this?

[pazeshun]

@knorth55 Oh, thank you for your hard work!
After this change, we have to change jsk_recognition_utils/__init__.py as well as package.xml when new dependency is added to chainermodels or datasets.
Is this OK for you?

[knorth55]

@pazeshun yes, probably it is ok.
also all chainer problems will be solved when all robots use noetic + python3...