Add missing error message for lttcomm_return_code
Fix: implicit conversion of enum types in consumer

Remove implicit conversions from enumeration type 'enum lttng_error_code'
to enumeration type 'enum lttcomm_return_code' found by clang.
Fix: clang 'constant-out-of-range-compare' warning

This patch fixes two 'autological-constant-out-of-range-compare'
clang warnings caused by comparing enum type variables with -1.

It's possible that compilers optimize these two comparsions away.