Flag non-existing configs on command line as errors

[deplinenoise]

Flag non-existing configs on command line as errors

[deplinenoise]

These errors are now flagged as missing units instead:

1087 ~/Sandbox/tundra$ tundra foo-bar
/Users/dep/Sandbox/tundra/scripts/boot.lua:71: couldn't find unit foo-bar
stack traceback:
    [C]: in function 'error'
    /Users/dep/Sandbox/tundra/scripts/boot.lua:71: in function 'errorf'
    /Users/dep/Sandbox/tundra/scripts/tundra/nodegen.lua:115: in function 'get_node_of'
    /Users/dep/Sandbox/tundra/scripts/tundra/nodegen.lua:83: in function 'generate'
    /Users/dep/Sandbox/tundra/scripts/boot.lua:739: in function 'generate_dag'
    /Users/dep/Sandbox/tundra/scripts/boot.lua:851: in function 'Build'
    [string "/Users/dep/Sandbox/tundra/tundra.lua"]:33: in main chunk
    (tail call): ?
    [C]: in function 'xpcall'
    /Users/dep/Sandbox/tundra/scripts/boot.lua:247: in function 'run_build_script'
    /Users/dep/Sandbox/tundra/scripts/boot.lua:885: in main chunk