JENKINS-16732 Catch and handle IOException when opening changelog

...or write

In the current build, if the plug-in failed to open the changelog file, the exception would rise high and fail the build.
Instead, just report the error and move on
(cherry picked from commit 6a01d2a426ce7b3db17c526c5df1afd8a846a8e7)

Signed-off-by: Guy Rozendorn <guy@rzn.co.il>