i2cslave: Fix missing translate() when raising exceptions

[notro]

The translate() changes where applied after travis had checked the
i2cslave PR so this wasn't caught.

[jerryneedell]

FYI - without this PR, the current master fails to build any of the ATMEL boards with errors like this

In file included from ../../shared-bindings/i2cslave/I2CSlave.c:40:0:
../../py/runtime.h:152:15: note: expected 'const compressed_string_t * {aka const struct <anonymous> *}' but argument is of type 'const char *'
 NORETURN void mp_raise_ValueError(const compressed_string_t *msg);
               ^~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
../../py/mkrules.mk:55: recipe for target 'build-circuitplayground_express_crickit/shared-bindings/i2cslave/I2CSlave.o' failed
make: *** [build-circuitplayground_express_crickit/shared-bindings/i2cslave/I2CSlave.o] Error 1
make: *** Waiting for unfinished jobs....

after including this PR -- all the builds succeed. I have only tried using the metro_m4_express and it seems OK.

[tannewt]

Thanks for following up! You rock as always.

[tannewt]

I'm going to merge this because it fixes everything but the translation build. Its verifying all strings are in the .po files for translators. You'll need to run make translate at the top level and do another PR. Thanks!