kunit: tool: fix display of make errors

CalledProcessError stores the output of the failed process as `bytes`,
not a `str`.

So when we log it on build error, the make output is all crammed into
one line with "\n" instead of actually printing new lines.

After this change, we get readable output with new lines, e.g.
>   CC      lib/kunit/kunit-example-test.o
> In file included from ../lib/kunit/test.c:9:
> ../include/kunit/test.h:22:1: error: unknown type name ‘invalid_type_that_causes_compile’
>    22 | invalid_type_that_causes_compile errors;
>       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> make[3]: *** [../scripts/Makefile.build:283: lib/kunit/test.o] Error 1

Secondly, trying to concat exceptions to strings will fail with
> TypeError: can only concatenate str (not "OSError") to str
so fix this with an explicit cast to str.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>