Having PlaybackManager start in Buffering was causing us to report
a media element readyState of HAVE_CURRENT_DATA. HAVE_CURRENT_DATA
doesn't make a whole lot of sense for local files, since we should have
all the data immediately when we process the metadata. This is
reflected in the buffered attribute, so let's not limit the ready state
unecessarily.
Use promises to await the expected sequence of events. Also, don't
assume that canplaythrough will fire after error. That depends on the
implementation.
If we fire the error event synchronously within the on_error callback,
then we'll end up destroying the PlaybackManager inside its own
callback and crash. Instead, queue a task to execute the error steps.
This could happen with or without MSE, but I observed it occurring on
YouTube with MSE when we hit a decoding error, since they immediately
try another source when an error is reported.
