I noticed a difference between how Plugin error messages are displayed in Webform and Entity Forms.
Webform: The error alert is shown right on top of the page and form

and on Entity Form it loads same area as form error notifications:

The markup difference Webform
<p class="text-danger"><span class="fa fa-exclamation-triangle" aria-hidden="true"></span> An unexpected error occurred from ISV code. (ErrorType = ClientError) Unexpected exception from plug-in (Execute): DummyPlugin.ThrowErrorPlugin: System.Exception: This is a dummy error </p>
vs. Entity Form which sit inside a MessagePanel div
<div id="MessagePanel" class="message alert alert-info alert-danger alert-danger" role="alert">
<span id="MessageLabel" tabindex="0"><p class="text-danger"><span class="fa fa-exclamation-triangle" aria-hidden="true"></span> An unexpected error occurred from ISV code. (ErrorType = ClientError) Unexpected exception from plug-in (Execute): DummyPlugin.ThrowErrorPlugin: System.Exception: This is a dummy error</p></span>
</div>