Improve error handling for ffmpeg operations
Improve error handling for ffmpeg operations

BUG=webrtc:7203

Review-Url: https://codereview.webrtc.org/2746413002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17258}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5c7a623dd45eff06bd173d9e32801188aa1eaf3c

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk