Handle NetworkInUse exception in api layer

In nova-network, if we delete an existing network with port already
associated, a 500 error will be reported and this is not helpful for
end user. This patch catch the network in use exception and raise
an HTTPConflict exception with formatted message.

Change-Id: I7361e461e0eecc0ef6e47fa8ef9a48a24b7d5dfd
Closes-Bug: #1294920
Merge "Handle NetworkInUse exception in api layer"