Bug 1844076: Only 1 error message shown even though there are 2 on "Site options"

- refactor networkingform & siteoptionform to not use _fail funcs
- siteoptionform now has its own validation func

behatnotneeded

Change-Id: I219ec6cbb56aa559b60a9cd97226601e2008305d
Signed-off-by: Lisa Seeto <lisaseeto@catalyst.net.nz>