raising proper exception for invalid allocation-pool and modifying display message for out of cidr gateway ip. This will fix 1349375
    https://bugs.launchpad.net/juniperopenstack/+bug/1349375
double commit in mainline.

Change-Id: Ifdea36aecc6f686feacb387b4e5f6d1cfeffb2e4
Merge "raising proper exception for invalid allocation-pool and modifying display message for out of cidr gateway ip. This will fix 1349375     https://bugs.launchpad.net/juniperopenstack/+bug/1349375 double commit in mainline."