Clarify the error codes in serialized_message.h

[clalancette]

https://github.com/ros2/rmw/blob/master/rmw/include/rmw/serialized_message.h is an extremely thin wrapper around rcutils_uint8_array_*. The problem with this is that users expect that these functions return RMW_* error codes, when in reality they return RCUTILS_* error codes. Even the documentation is confused about this; it is documented as returning RMW_* error codes, which is just incorrect.

My suggestion is that we make these static inline functions instead, with a conversion between the RCUTILS error codes that are returned from the rcutils_uint8_array functions and the RMW error codes that they are documented to return.