Add a workflow step that allows setting the unsuccessful message from a
workflow build.  config.jelly is currently missing.

Dependency to the workflow plugin is optional.

Implements a limited part of [JENKINS-26103].

Extend the gerritReview workflow step to also allow setting the URL to
post.  Use the existing mechanism (BuildMemory) for storing the value
while the build is running.

Implements a limited part of [JENKINS-26103].

Rename a few methods from "failure message" to "unsuccessful message"
for better consistency.  One older public method is still kept as
deprecated and forwards calls to the new method.
Similar renaming also in comments and tests.

- Reduce some logging levels.
- Adjust log message to match both workflow and freestyle job behavior.

This is a better name for the current functionality, and it is probably
better to add additional functionality as separate steps instead of
making a single step that does everything.