NSX: Do not extend fault map for network gateway ext

Most of the exceptions defined for this extension already extend
an exception-mapped class. This patch ensures this is true for
every exception and then removes the fault map extension code.

Change-Id: Ifdd9b6d3892cff2d076bdab3a2a1252923e3f85d
Closes-Bug: 1319423
Merge "NSX: Do not extend fault map for network gateway ext"