Throw an exception if LuaSandboxFunction::call returns false

If LuaSandboxFunction::call returns false, it's an error on PHP's part.
Throw a "real" exception so that we can see what's causing it in server
logs.

Bug: 71045
Change-Id: I7185e186d3e0af6e467b73ea1ef13417ca96b088
Merge "Throw an exception if LuaSandboxFunction::call returns false"