Clear out pending errors on player disposal.

Source selection errors are dispatched asynchronously so that there is an opportunity to override the error message. If the player is disposed during that period, the error timeout wasn't being cleared properly. Fix for #1480.
I encountered this issue originally through calling `src()` after the player was constructed so I reproduced that scenario for my test case. I'm fairly certain the same error would occur if the sources came in through options. Do you want me to adjust the test?