src/install: proper error handling for resolve_loop_device()
src/install: proper error handling for resolve_loop_device()

This adds GError reporting to resolve_loop_device to allow propagating
failure messages.

Changing the corresponding call in determine_slot_states() to check for
the return value of resolve_loop_device() also fixes the issue that
despite resolve_loop_device() can return NULL, the return value was
directly used as input for find_config_slot_by_device() (failed only at
method input parameter verification then).

While at it, also adapt the tests to print out an error message in case
of failure.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc