Improve GN duplicate name error.

Improve GN duplicate name error.

Improve messaging when two targets generate the same output.

Previous message:
  ERROR Duplicate rules for libfoo.so.TOC

New message:
  ERROR at //BUILD.gn:24:1: Duplicate output file.
  shared_library("b456") {
  ^-----------------------
  Two or more targets generate the same output:
    libfoo.so.TOC
  This is normally the result of either overriding the output name or
  having two shared libraries or executables in different directories
  with the same name (since all such targets will be written to the root
  output directory).

  Collisions:
    //:b456
    //:a123

  See //BUILD.gn:21:1: Collision.
  shared_library("a123") {
  ^-----------------------

Committed: https://crrev.com/bfd2deab3b5c8dea29671662116081a3878687a4
Cr-Commit-Position: refs/heads/master@{#349521}

[brettw, 2015-09-17 22:02:48]

brettw@chromium.org changed reviewers:
+ jam@chromium.org

[brettw, 2015-09-17 22:03:01]

The CQ bit was checked by brettw@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-17 22:03:57]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1350173002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1350173002/1

[jam, 2015-09-17 22:07:06]

lgtm, thanks

[brettw, 2015-09-17 22:11:32]

The CQ bit was unchecked by brettw@chromium.org

[brettw, 2015-09-17 22:12:08]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2015-09-17 22:13:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1350173002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1350173002/1

[commit-bot: I haz the power, 2015-09-17 22:26:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-17 22:27:53]

Patchset 1 (id:??) landed as
https://crrev.com/bfd2deab3b5c8dea29671662116081a3878687a4
Cr-Commit-Position: refs/heads/master@{#349521}