More useful error messages when a workflow plugin is missing

Currently if a workflow is using one of the functions from a plugin like JIRA Suite Utilities or JIRA Misc Workflow Extensions and the JIRA instance is moved to a different server (upgrade, migration, etc.) where that plugin isn't installed, when an issue is attempted to be created in that workflow, the user sees an extremely unhelpful error message:

Error creating issue: Could not load FunctionProvider class

There's absolutely no indication of what this actually means. Looking within the workflow at the step that's causing the problem will yield something like this:

Type: class
Class: com.googlecode.jsu.workflow.function.CopyValueFromOtherFieldPostFunction

Adding that information to the "Could not load FunctionProvider class" error above would allow people to realise themselves at least that something is missing, and an indication of what it might be.