The mm-renderer service stores information about any error that occurs when an API call is made. When requested, this error information is returned in a structure that includes a code for the mm-renderer error type and other information specific to either the media protocol used or to the underlying POSIX error.
It's good practice to check for errors after each API call, to promptly learn of any problems with the media configuration or playback. You can do this by examining each function call's return code and if it indicates failure (e.g., by a non-zero value, often -1), calling mmr_error_info() to get information about the error that occurred. The action to perform following a given error type is a design choice.