Use R -e 'stopifnot(all(data.frame(devtools::test())$failed == 0L))' to capture errors
Use R -e 'stopifnot(all(data.frame(devtools::test())$failed == 0L))' to capture errors

and add ffmpeg install to fix R build failure.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io