To fix this, you must associate the ActiveMQ XSD URL with the schema.
Go to XML->XML Catalog in Preferences, and add a User Specified Entry.
Location: http://activemq.apache.org/schema/core/activemq-core-5.3.0.xsd
Key Type: Namespace Name
Key: http://activemq.apache.org/schema/core
Then add a second one:
Location: http://activemq.apache.org/schema/core/activemq-core-5.3.0.xsd
Key Type: Schema Location
Key: http://activemq.apache.org/schema/core/activemq-core.xsd
Hit OK.
If you have this same error and Tomcat fails, open the applicationContext-jms.xml file which triggers the error and replace:
http://activemq.apache.org/schema/core/activemq-core-5.2.0.xsd
with
http://activemq.apache.org/schema/core/activemq-core.xsd