ELInstance.getErrors() is better solution.
But I've noticed another thing:
among the following EL strings

<h:outputText value="#{msgs['prompt'}" />
<br> </br>
<h:outputText value="#{msgs[greeting']}" />
<br> </br>
<h:outputText value="#{msgs'[prompt']}" />
<br> </br>
<h:outputText value="#{msgs[greeting]}" />

EL "#{msgs[greeting]}" (without quotes) does not have SyntaxError entity. That's why this EL is substituted with the bundle version.

So could EL parser indicate that the quotes are missing at all?

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