lorawan: Cleanup error handling code

The error handling code currently has a couple of issues:

 * It relies on ordered lists and upstream not changing any constants.

 * Converted messages are not stored in constant memory which means
   that log_strdup is needed whenever they are printed.

This change also factors out error handling to a separate file,
lw_priv.{c,h}, to facilitate reuse in a future secure element and
state storage implementation.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>