Add errors for various chooser failure modes, and remove assertions that are about to change.

Add errors for various chooser failure modes, and remove assertions that are about to change.

NoBluetoothChooser
ChosenDeviceVanished
ChooserCancelled

1st of 3 patches:
1. This patch.
2. Wire up the chooser on the Chrome side. (https://codereview.chromium.org/1286063002/)
3. Update the test assertions and remove now-unused errors. (https://codereview.chromium.org/1284143006/)

BUG=500989
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201175

[Jeffrey Yasskin, 2015-08-14 22:54:49]

jyasskin@chromium.org changed reviewers:
+ ortuno@chromium.org, scheib@chromium.org

[ortuno, 2015-08-17 16:38:47]

lgtm

[scheib, 2015-08-25 18:09:13]

LGTM

[Jeffrey Yasskin, 2015-08-25 23:28:45]

The CQ bit was checked by jyasskin@chromium.org

[commit-bot: I haz the power, 2015-08-25 23:28:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293593003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293593003/1

[commit-bot: I haz the power, 2015-08-25 23:30:42]

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

[commit-bot: I haz the power, 2015-08-25 23:30:43]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[Jeffrey Yasskin, 2015-08-25 23:35:24]

The CQ bit was checked by jyasskin@chromium.org

[Jeffrey Yasskin, 2015-08-25 23:35:25]

The patchset sent to the CQ was uploaded after l-g-t-m from scheib@chromium.org,
ortuno@chromium.org
Link to the patchset: https://codereview.chromium.org/1293593003/#ps20001
(title: "Sync")

[commit-bot: I haz the power, 2015-08-25 23:35:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293593003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293593003/20001

[commit-bot: I haz the power, 2015-08-26 00:39:11]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201175