lp8 loader: Ensure that lp8 path macro is defined.

Previously, the macro `8680/lua-pack/path`, which contains the path of
the lp8 to be loaded, may have been undefined at the call site of the
macro `NX_LOAD_LP8` if the macro `NX_LP8` was defined as `1.1`,
resulting in a preprocessor error (“macro/file "8680/lua-pack/path" is
missing at lua/lp8/_main.cfg:16”).
lp8 loader: Improve resilience and verbosity.

Add extra error checks.
lp8 loader: Allow prioritization of internal lp8.

Add `NX_LP8` code `1.2` (use internal lp8 if available, else use
external lp8), and reassign `NX_LP8` code `1.1` (use external lp8 if
available, else use internal lp8) to `2.1`.