Add error reporting on VP8 encoder configuration error

Bug: webrtc:11310
Change-Id: I4ceb565b211a2313add193a3859f3baeaacc3e87
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/182001
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31964}

modules/video_coding/codecs/vp8/libvpx_interface.h

@@ -93,6 +93,8 @@ class LibvpxInterface {
       vpx_codec_ctx_t* ctx,
       vpx_codec_iter_t* iter) const = 0;
 
+  virtual const char* codec_error_detail(vpx_codec_ctx_t* ctx) const = 0;
+
   // Returns interface wrapping the actual libvpx functions.
   static std::unique_ptr<LibvpxInterface> CreateEncoder();
 };