Handle time source exception

[mjforan]

When playing data from a bag I get this error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  can't subtract times with different time sources [1 != 2]
Aborted (core dumped)

In my case this stems from the section of image_transport_display.hpp which computes the topic frequency.

My change catches this exception and makes it a warning on the topic property instead. If approved, I will apply the change to other applicable display interfaces.

[ahcorde]

Pulls: #1285
Gist: https://gist.githubusercontent.com/ahcorde/4bd58742fe756b64abe70574694d7abc/raw/e98f6ea463e22f33f6891e3e20be5b24f772ba89/ros2.repos
BUILD args: --packages-above-and-dependencies rviz_default_plugins --packages-above-and-dependencies rviz_default_plugins
TEST args: --packages-above rviz_default_plugins --packages-above rviz_default_plugins
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/14686

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