Fix error handling in X11 screen capturer
Fix error handling in X11 screen capturer

Previusly errors from XServerPixelBuffer::CaptureRect() were not always
handled, which results in a black frame returned from the capturer
instead of an error.

BUG=webrtc:7305

Review-Url: https://codereview.webrtc.org/2738513005
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17101}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9fa7e4a0ac20b89f6c147f27a7e04a762378aaa4

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