player: add an option to abort playback on partial init failures

This is probably what libmpv users want; and it also improves error
reporting (or we'd have to add a way to communicate such mid-playback
failures as events).

DOCS/client-api-changes.rst

@@ -28,6 +28,8 @@ API changes
  1.9    - add enum mpv_end_file_reason for mpv_event_end_file.reason
         - add MPV_END_FILE_REASON_ERROR and the mpv_event_end_file.error field
           for slightly better error reporting on playback failure
+        - add --stop-playback-on-init-failure option, and make it the default
+          behavior for libmpv only
  1.8    - add qthelper.hpp
  1.7    - add mpv_command_node(), mpv_command_node_async()
  1.6    - modify "core-idle" property behavior

DOCS/man/options.rst

@@ -252,6 +252,12 @@ Playback Control
 
         Without ``--hr-seek``, skipping will snap to keyframes.
 
+``--stop-playback-on-init-failure=<yes|no>``
+    Stop playback if either audio or video fails to initialize. Currently,
+    the default behavior is ``no`` for the command line player, but ``yes``
+    for libmpv. With ``no``, playback will continue in video-only or audio-only
+    mode if one of them fails. This doesn't affect playback of audio-only or
+    video-only files.
 
 Program Behavior
 ----------------