settings: do not panic on backend initialization errors

There is little reason to panic on settings backend initialization error.
Such behavior was introduced with initial settings subsystem support, which
was adapted from MyNewt. This is not the usual way how Zephyr handles
errors, so it is time to change that.

There is already handling of some errors by simply returning / propagating
them to caller. Rework all the paths that resulted in k_panic() to also
return error codes.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>