I'm writing a simple MockStrutsTestCase and getting either an Error or a Failure. How does JUnit define the difference between these - Error vs Failure?

Error:

servletunit.struts.ExceptionDuringTestError: A NullPointerException was thrown. This may indicate an error in your ActionForm, or it may indicate that the Struts ActionServlet was unable to find struts config file. TestCase is running from C:Eclipseworkspacemyproject directory. Context directory is C:Eclipseworkspacemyproject. struts config file must be found under the context directory, the directory the test case is running from, or in the classpath.

Failure:

junit.framework.AssertionFailedError: Error initializing action servlet: Unable to find /WEB-INF/web.xml. TestCase is running from C:Eclipseworkspacemyproject directory. Context directory is C:EclipseworkspacemyprojectWEB-INF. /WEB-INF/web.xml must be found under the context directory, the directory the test case is running from, or in the classpath.

Vasyly