Improve error message in navigator.requestMediaKeySystemAccess()

Improve error message in navigator.requestMediaKeySystemAccess()

BUG=480141
TEST=compiles

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197312

[jrummell, 2015-06-17 21:40:03]

jrummell@chromium.org changed reviewers:
+ ddorwin@chromium.org, sandersd@chromium.org, xhwang@chromium.org

[jrummell, 2015-06-17 21:40:04]

This appears to be the only unfriendly EME promise rejection in blink. Not sure
how we can make it actionable, as it only gets hit in fuzzer tests.

[xhwang, 2015-06-17 21:42:29]

lgtm

[jrummell, 2015-06-17 22:15:41]

The CQ bit was checked by jrummell@chromium.org

[commit-bot: I haz the power, 2015-06-17 22:16:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1191033002/1

[commit-bot: I haz the power, 2015-06-18 01:59:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-18 01:59:30]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/67114)

[jrummell, 2015-06-18 02:04:56]

The CQ bit was checked by jrummell@chromium.org

[commit-bot: I haz the power, 2015-06-18 02:05:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1191033002/1

[commit-bot: I haz the power, 2015-06-18 03:04:06]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197312