GN: fail gracefully when git commit info cannot be extracted

See https://groups.google.com/a/chromium.org/d/msg/gn-dev/7nlJv486bD4/V9ZsfV_nBAAJ
for context.

BUG=551343
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/2237913002 .

Cr-Original-Commit-Position: refs/heads/master@{#412213}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e9fcf209818dede2348c41913994b5d9ea81314c
diff --git a/tools/gn/last_commit_position.py b/tools/gn/last_commit_position.py
index b710ed0..a91f722 100644
--- a/tools/gn/last_commit_position.py
+++ b/tools/gn/last_commit_position.py
@@ -95,7 +95,6 @@
 
 value = FetchCommitPosition(git_directory)
 if not value:
-  print "Could not get last commit position."
-  sys.exit(1)
+  value = 'UNKNOWN'
 
 WriteHeader(output_file, header_guard, value)