From Cisco IOS Doc
<http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca7a7.html#wp1000967>
"A FAIL response is significantly different from an ERROR. A FAIL means that
the user has not met the criteria contained in the applicable authentication
database to be successfully authenticated. Authentication ends with a FAIL
response. An ERROR means that the security server has not responded to an
authentication query. Because of this, no authentication has been attempted.
Only when an ERROR is detected will AAA select the next authentication
method defined in the authentication method list."
Vijay Avarachen