Hook up the error signals in the L0 cache

[LucaRufer]

The current implementation of the L0 has an in_error_o signal, which is just tied to zero. The error signal provided by the refill/L1 is ignored.
In this PR, the error signal is hooked up in the L0 the same way it is hooked up in the L1.

Changes:

• The icache handler forwards error signals on cache line hits from the L1 to L0.
• The L0 gets an error bit per cache line indicating if an error occured while fetching the cache line (same as in L1).
• On an L0 hit, the error signal is provided together with the data.

[colluca]

Thanks for the contribution @LucaRufer!

Tomorrow I will test it in Snitch before merging, but otherwise looks good to me.