[Telemetry] Some fixups in speed index calculation.

tools/telemetry/telemetry/core/platform/android_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/android_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/android_platform_backend.py b/tools/telemetry/telemetry/core/platform/android_platform_backend.py
index 755142e4a5112e1c920d1d919f037f6410c2d775..64c8f645c1a32d5ddc20604bf48c99d5dc9d8440 100644
--- a/tools/telemetry/telemetry/core/platform/android_platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/android_platform_backend.py
@@ -222,13 +222,17 @@ class AndroidPlatformBackend(
 
     def GetDimensions(video):
       proc = subprocess.Popen(['avconv', '-i', video], stderr=subprocess.PIPE)
+      dimensions = None
+      output = ''
       for line in proc.stderr.readlines():
+        output += line
         if 'Video:' in line:
           dimensions = line.split(',')[2]
           dimensions = map(int, dimensions.split()[0].split('x'))
           break
       proc.wait()
-      assert dimensions, 'Failed to determine video dimensions'
+      assert dimensions, ('Failed to determine video dimensions. output=%s' %
+                          output)
       return dimensions
 
     def GetFrameTimestampMs(stderr):