Emit contextual error message during incremental m2e builds

[kwin]

This closes #1960

Please DO NOT FORCE PUSH. Don't worry about messy history, it's easier to do code review if we can tell what happened after the review, and force pushing breaks that.

Please make sure that your PR allows edits from maintainers. Sometimes its faster for us to just fix something than it is to describe how to fix it.

After creating the PR, please add a commit that adds a bullet-point under the [Unreleased] section of CHANGES.md, plugin-gradle/CHANGES.md, and plugin-maven/CHANGES.md which includes:

• a summary of the change
• either
  • a link to the issue you are resolving (for small changes)
  • a link to the PR you just created (for big changes likely to have discussion)

If your change only affects a build plugin, and not the lib, then you only need to update the plugin-foo/CHANGES.md for that plugin.

If your change affects lib in an end-user-visible way (fixing a bug, updating a version) then you need to update CHANGES.md for both the lib and all build plugins. Users of a build plugin shouldn't have to refer to lib to see changes that affect them.

This makes it easier for the maintainers to quickly release your changes :)

[nedtwigg]

Good start :)

[nedtwigg]

Great! This just needs changelog entries in CHANGES.md and plugin-maven/CHANGES.md and then it's ready to merge.