Improve disable_functions handling in LuaStandalone

If the user is on a webhost that has proc_open listed in PHP's
disable_functions directive, we should give a better error message.
Until we no longer support PHP below 5.4, we should do the same for
safe_mode. And since we're doing that, we may as well report any other
warnings if proc_open fails, too.

In addition, this cleans up error handling in
Scribunto_LuaEngine::load() so it doesn't pretend the interpreter is
loaded if getInterpreter() throws an exception. Otherwise other code
winds up with PHP fatal errors trying to access a null value.

Bug: 50706
Change-Id: I2887b722e089fd7a526aa7dcab9c80deb343d8ac
Merge "Improve disable_functions handling in LuaStandalone"