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") {
  ^-----------------------

Review URL: https://codereview.chromium.org/1350173002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349521}

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