Add missing error for non-'+' string op.

Compiler was returning an error message without printing an error.

Future considerations: there are a lot of other errors the compiler
could be throwing here, for other things that are wrong, without
aborting early.

Fixes: 169628569
Test: aidl_parser_fuzzer tests/corpus/other_string_operation
Change-Id: I1131e8c530cf7c562a7af68e5036796f44976852
2 files changed